Thumb-sucking is an Oral Fixation associated with babies. Normally, most children grow out of the habit. Some older children and adults however keep doing it, much to the embarrassment of others. In comedy this will be Played for Laughs to indicate that this person is a Manchild who never grew up. In drama this trope is used more to indicate a person's insecurity and/or to present him as cute and pitiful.
Thumbsucking with adults is Truth in Television too, as some people still can't shake the habit off as they grow older, usually as a result of some trauma or feelings of stress, anxiety and/or insecurity.
- In Violine, the ten-year-old main character is seen sucking her thumb while sleeping.
- Peanuts: Linus, a small boy, is often seen sucking his thumb while holding his Security Blanket, a blanket he needs to feel comfortable.
- Pocky In Ramen. Matt still sucks his thumbs.
- Thumbsucker is a tragicomedy about a teenager who still sucks on his thumb.
- Turkish Delight. Olga, Erik's love interest, still sucks on her thumb. Here it's played to show what a cute lovable woman she is.
- The Client. The film starts off with two boys witnessing a Mafia murder in the forest. The youngest is so severely traumatized that he lies in bed sucking his thumb.
- In the second part of Kill Bill Beatrix meets Esteban, who tells her:
Esteban: Ahh... You must be Beatrix. I can see the attraction. I remember when Bill was only five years old, I took him to the movies. It was a movie starring Lana Turner. The Postman Always Rings Twice with John Garfields. And whenever she would appear on the screen, Bill would begin to suck his thumb to an obscene amount. And I knew from this very moment, that this boy was a fool for blondes. Mmm...
- In Dumb and Dumber (1994), Lloyd is showing sucking his thumb after Harry knocks out the would-be rapist Seabass with the stall door.
- Ida's character in Motel Hell is introduced sucking her thumb while sleeping. Said character is almost forty years old.
- In Guardians of the Galaxy Vol. 2, one of the Ravagers on Yondu's ship, the Eclector, is seen sucking his thumb in his sleep.
- Struwwelpeter: One of the more infamous stories shows what happens to little boys who suck their thumbs: a man will appear out of nowhere with a cutting device and cut your thumbs off.
- In The Naked Sun, Solarians have extreme aversion to the physical presence of other people, and only communicate through holograms. When Baley communicates with one Solarian who's particularly bad that way, and states his intention to come into his personal presence, the Solarian becomes hysterical to the extent of sucking his thumb. Baley notes it's as if he's regressing to an age when he still could see other people — under five.
- In Hogfather, Peachy is a member of a gang of thugs who are specialized in theft and murder. His childhood fear is the Scissor Man, a monster that comes after children who suck their thumbs. Late in the book, when Peachy and the rest of the gang are in the Toothfairy Tower, the childishness-inducing effect of the Tower starts taking hold on him, and the thumb sucking starts to resurface. Then the Scissor Man manifests in physical form and it's implied that he brutally murders him.
- Happy Days: In one episode, Marion notes that she can tell when any of her teenage children are in a bad mood because Richie goes into a sulk and Joanie and Chuck suck their thumbs.
- M*A*S*H. Hawkeye implies this when telling B.J. a series of embarrassing stories in order to get him to say what his initials really stand for; whether or not they're really true or not is open to interpretation.
Hawkeye: Okay, you wanna hear a really embarrassing story? I sucked my thumb till I was 21. I gave it up for bedwetting. Pretty hot stuff, huh? So, what does B.J. stand for?
- Mork & Mindy: Whenever Mork feels sad or scared he starts sucking his thumb.
- In Dad's Army, Pike is shown to still suck his thumb despite being old enough to be conscripted (although his rare blood type means he doesn't get signed up).
- In the Murdoch Mysteries episode "Me, Myself, and Murdoch", a twenty-something young woman is described as sitting on a swing sucking her thumb, and this behavior is given as evidence of her mental incapacity. It turns out to be a cue for one of her multiple personalities (that of a three-year-old girl) brought on by her witnessing her father dismembering her mother's body when she was three years old.
- In an episode of NYPD Blue Detective Kirkendall's son (circa 8 years old) witnesses a murder. After he's told the police what he knows he's taking a nap in the station house's bunkbed, with his mother cradling his head in her lap. While he sleeps, his thumb goes into his mouth. She gently removes it, but a minute later it goes back in again and she lets it be.
- Saved by the Bell: When Zack, Screech and Slater have been out late at night at an adults-only club (they snuck in with Fake IDs), Screech is seen the next day dozing in the hallway while sucking his thumb.
- The singer of "Fuck You" by Archive suggests the person he loathes still sucks his thumb as a way to demean them.
Bet you sleep, like a child, with your thumb in your mouth. I could creep up beside and put a gun in your mouth.
- In Super Smash Bros. Donkey Kong sucks his thumb when he falls asleep.
- One way that the mother in When Latina Moms Embarrass You In Front Of Your Friends embarrasses her daughter is by loudly telling someone on the phone that she still sucks her thumb, while her friends are in earshot at that.
- Time Squad: One episode has the heroes visit Billy the Kid who acted like a genuine big baby, sucking his thumb and walking around in a diaper.
- Kim Possible: In the episode "Emotion Sickness". Drakken curls up in the Troubled Fetal Position and starts sucking his thumb after dealing with Shego's Phlebotinum-induced mood swings.
- Arthur: "All Thumbs" deals with Buster all of the sudden having a problem sucking his thumb that he never had before, and how Arthur makes a big deal out of it, because he shouldn't still be sucking his thumb at eight-years-old.
- In The Fairly OddParents!, the Crimson Chin in his debut episode began thumb sucking during his Heroic BSoD when he learned that he is fictional.
- In one episode of Kaeloo, Mr. Cat tries to convince the others that he's still a childnote by sucking his thumb.
- in Welcome to the Wayne premiere "Rise and Shine Sleepyhead" Saraline Timbers falls back into this habit, despite a monologue trying to work up her will not to.
- Little Princess: When the main adult cast are feeling indignant at the end of "I Want My Dummy" that they have to give up several of their hobbies (they'd promised to give something up if the Princess promised to give up their pacifier), they all suck their thumbs.