A speech impediment that strikes vampires (and other fanged creatures, as well), where their spoken s's become th's or, more often, sh's.
Here's the technical explanation for why it happens: False teeth tend to cause sibilants (s; soft c, like the first c in "circus"; and sometimes z) to be mispronounced because the prosthetics force a change in the position of the tongue. Interdentals (th) are made by the tongue going against the upper incisors - sibilants are more likely to be mispronounced as interdentals when the incisors are altered or missing, as with fake buck teeth or missing front teeth note More common with prosthetic fangs are for the sibilants to become post-alveolar fricatives (sh), caused by the tongue being forced back by the wider and longer canines. For example, listen to anything The Master says in Buffy season one. When he comments, "Nice dress", he pronounces it "Nishe dresh", not "Nithe dreth".
Fang Thpeak can be called to the audiensh'sh attention in two waysh:
- When the writersh make no effort to avoid shibilantsh, charactersh who have fangsh will alwaysh shpeak thish way, or,
- When the people writing the show, having one actor (or more) who wear a fang proshthetic, go out of their way to try to write dialogue avoiding the letter "S". An attempt of that nature will very likely end with the character having a rather odd vocabulary and pattern of word choishe. And the shibelentsh that do get through are glaring.
Can be avoided by the actors just redubbing the offending voices over later (without the problematic prosthetics), but episodic tv shows don't always have the time or the budget for that.
The same thing happens to a slightly lesser degree to people who wear upper dentures, and orthodontic retainers. Again, it's the mouthpiece interfering with the position of the tongue.
Other consistent mispronunciations include ventriloquists substituting g or v for b, and f for p — it's nearly impossible to pronounce a b or p sound without noticeable movement of the lips.
- Mr. Scarface in Batman comics had this problem, pronouncing all 'B's as 'G's. This was because he was actually a ventriloquist dummy who was the split personality of Arnold Wesker (appropriately titled the Ventriloquist), and Batman has used it to figure out his identity a few times when Scarface was trying to pretend he was somebody not made of wood. This speech impediment was not carried over to Scarface's appearances in Batman: The Animated Series or The Batman, since it was deemed too awkward.
- Many kids in the Asterix comics speak like this, including kid-ified versions of most of the main cast during the Flashback story How Obelix Fell into the Magic Potion.
- Krurgor in Les Naufragés d'Ythaq belongs to a race that can basically be described as bipedal walrusses, including speech-hindering fangs.
- Another fang-induced lisp is that of the Cafou, a giant, talking sabertoothed cat in Les Lumières de l'Amalou.
- In Empowered, the villain King Tyrant Lizard (a human turned intelligent dinosaur) and the hero Homunculoid talk like this; the former has a tyrannosaur's jaws and teeth, the latter has oversized lips and tongue.
- The Beast in Fables, whenever his curse is only partially in effect — he's got fangs but not a big enough mouth to fit them in, so they get in the way.
- Mooch the cat in Mutts. He has a speech impediment that causes him to insert an sh into words when he "shpeaks", such as yesh (yes), shmilk (milk), and shmousie (mousie).
- In Calvin and Hobbes, Calvin mentions that Sabre Tooth Tigers probably did this.
- In The Multiversity, this is how the snake Sivana speaks, very much averting Sssssnake Talk.
- Chrysalis Visits The Hague has a bad grammatical case of this; It's largely justified when ponies are holding platters and swords in their mouths, but that doesn't explain why any character's speech is reduced to comical 'shlurs' when they have as much as a single cigarette between their lips. Edith's subtler slurring is justified at least, since a falling rock almost ends up breaking her jaw.
- Planet of the Apes (2001) fell into this with a lot of the ape actors. Tim Roth had it particularly bad, and his character's name was Thade. That's rough.
- WNUF Halloween Special has a scene where a costumed audience outside of a haunted house is being asked about the house's history. A tall, scared-looking man in a vampire costume, complete with cheap plastic fangs, says, "Shomebody died in thish houshe?"
- Lampshaded in Red Dragon, particularly the book. Dolarhyde has a corrected harelip and cleft palate and avoids sibilant words, for example, always using "Mmm-hmm" instead of "Yes."
- It's not vampires that suffer from this in Discworld; rather, it's their servants, the Igors (and Igorinas), though they can turn it off to be understood better. This varies from Igor to Igor. The hip young Igor who works for the City Watch has it intermittently (his relatives call it a thpeech impediment), while another Igor was able to shut it off for a complex explanation. Given the nature of Igorhood this could have something to do with what dental surgery a particular Igor has dabbled in.
- Vimes believes people in drought-stricken countries would pay good money for Igor to say "sausages".
- Offler the crocodile god has this problem due to, well, having a crocodile's head and so has an impressive set of uniform teeth and no proper lips.
- Gargoyles have their mouths stuck permanently open, and talk in a manner that reflects this.
- Angua of the City Watch and other werewolves speak like this mid-way through the change into wolf, due to elongated fangs.
- Cribbins, from Making Money, talksh like thish due to the poorly-fitting dentures he stole from a man just before killing him.
- Betsy Taylor in Mary Janice Davidson's Undead and Unwed thpeakth like thith.
- Vargo Hoat from A Song of Ice and Fire. He's a normal human, but monstrous in morality and Laughably Evil.
- Buffy the Vampire Slayer and Angel. In the DVD commentary for the first season of Buffy, Joss Whedon explains that the prosthetic fangs the actors wear make particular sounds difficult to pronounce properly. The makeup artists and the actors learned a few tricks to get around this; custom prosthetics for major characters eased this problem. Whedon even invoked this trope in the first season Buffy DVD commentary by apologizing to one of his guest-stars for making him use the phrase "excruciating loser" in full vamp-face. Juliette Landau wore her fangs for an extended period before taking up the role of Drusilla specifically to learn how to speak properly with them. Unfortunately, guest vamps lacked the time for this and some had more trouble than others.
- Many of the Klingons and Ferengi on Star Trek's various series, particularly since they typically had entire mouths full of awkward prosthetic teeth. Particularly noticeable when recurring actors had to wear the makeup — such as in the episodes of Deep Space Nine where Odo, Sisko, and O'Brien go undercover as Klingons. O'Brien even points it out. And Michael Dorn (who played Worf, a Klingon, for at least 13 years) found their plight rather amusing.
- Armin Shimerman noted that the hardest word for him to pronounce with the Ferengi fangs was "hellhole".
- On an aside, one of the more common Klingon expressions seen on-screen is teeth-grinding for annoyance. One imagines that at least some of that annoyance is method-acting.
- Vir Cotto from Babylon 5 originally sported large canine teeth like all other Centauri, but they quickly disappeared because it made the actor talk like this.
- In Chinese Paladin 3, the temporarily-vampirized hero has a huge problem with this. Not only does he struggle obviously and markedly with pronounciation, he often sprays innocent bystanders with spit in the process.
- Not so much in True Blood (although it does happen) but in the parody video of the show.
- The Cat on Red Dwarf suffers a little bit of this from time to time.
- Supernatural. Spoofed in "LARP And The Real Girl" when a LARPer dressed as an orc has his false fangs fall out in the middle of a slurred Badass Boast. As he's held in the stocks, someone else has to put them back in for him.
- Vampire: The Masquerade Bloodlines has a few examples that make you wonder whether the VA was wearing prosthetics, among the youngest of the vampires.
- All of the Trolls in Homestuck have fangs, but Sollux is the only one that has been stated to have a lisp. This is because Sollux is also the only one with doubled incisors (and it is only the incisors, and their counterparts in the lower jaw, that are doubled). Which is why it vanishes when Karkat accidentally knocks those extra fangs out.
TA: lips, lips, lipsssss. w0w, it feels s0 great t0 say that w0rd!
- Similar to the example from Homestuck, members of Species X from DNA have mouths full of sharp teeth but only one of them, named Orion, talks like this, and he gets picked on for it. His teeth stick out noticeably more than the rest of the Xs'.
- Trope-tan, in The Way of the Metagamer, lisps occasionally due to her Cute Little Fang.
- Camille, Shannon's fragile little sister from Bloody Urban has crooked, front-facing fangs which result in a speech impediment.
- Iacar from Wurr haf a diftinctive lifp.
- The Jagermonsters in Girl Genius show enormous fangs when they speak or smile, some even have tusks, but for reasons somewhere between plot development and Rule of Funny, they speak with Funetik Aksent - cod-German or East European, in their case. The harshness of the accent could be put down to the fangs, but the handful of Jagers without these dental excesses talk the same way, so it seems to be cultural.
- Wapthi Thquare, uh, Wapsi Square: Monica ekthperientheth a temporary(?) bout of this as a result of her Jaguar Girl identity coming to the fore in the Bibliothiki.
- Shelly Marsh, from South Park, speaks like this because of her braces and retainer.
- Penny from ChalkZone.
- Beth from Total Drama had this is season one, because she had braces.
- Phineas gets this when he dresses as a vampire for Halloween.
- The protagonist of Last Year's Snow Was Falling speaks that way (Lost in Translation, unfortunately). No explanation is given, but the condition is permanent. Possibly a parody on the Soviet leader Leonid Brezhnev, who suffered from throat cancer.
- A Pup Named Scooby-Doo mixes this with Vampire Vords, where an actor who plays a vampire claims that the fake fangs he wears makes him speak in an accent when asked why he doesn't have an accent off-screen.
- A common result of wearing ordinary non-fang dentures, Sean Connery's distinctive speech being a notable example.
- Anyone with a freshly pierced tongue, lip, or cheek tend to talk like this until they heal, as will anyone who's just had dental work and still has numb lips from the anesthetic.
- Talking while wearing a dental retainer can often cause this, especially if the retainer is new and you're not used to it yet.
- Ermahgerd! Gersberms! Mah favrit berks! 
- Try speaking a language that isn't your native one right after getting braces. Not pretty.
- Legendary Notre Dame Fighting Irish coach Lou Holtsh.
- Germans trying to speak English may thometimes do this as a form of overcompensation.
- Listening to a recording of people speaking, which has a lot of static in it, may, for some reason, also sound like everyone has a lisp.
- Sometimes this trope may be just linguistic, as in certain Spanish dialects. The "official" pronunciation in Spain is the Castilian one, where the soft "c" sound is pronounced "th" as in "thin" rather than "s". So "cero" (zero) would sound like "thero", and "cazador" (hunter) would sound like "cathador," and so on. Ironically, this is the least common pronunciation — the southern half of Spain and the Spanish-speaking countries of South and Central America mostly use the "s" sound ("sero", "casador"...). More on this on Wikipedia.
- Same for Portuguese accent. In Portuguese many words are written like in Spanish, but pronounced differently. This includes "sh" in place of "s".
- Any vampire LARPG player who's just gotten a new set of fake fangs tends to lisp slightly until they get used to the elongated incisors.