An attack at someone's mouth is one of the most intense ways to hurt or kill a person.
The mouth is one of the most sensitive parts of the body. It's where we repeatedly let foreign objects enter our body — it's just that usually those objects are nourishment of some kind. Sticking in something dangerous will likely create visceral feelings of horror and revulsion in people. It has shades of Body Horror even when that trope isn't actually happening. And being situated in the head, it's also very close to the brain,note and thus presents an alternate direction for targeting the body's most vital organ.
This is why any kind of attack there, whether it's putting the barrel of a gun in someone's mouth, making someone unknowingly ingest poison, or an Orifice Invasion through the mouth, will likely be to show just how serious things have gotten.
This is largely a reason why the Depraved Dentist trope still happens even with modern medicine, since the thought of someone doing things in one's mouth is not pleasant for many people.
In fiction this can be portrayed as a way for villains to get some fitting comeuppance. Perhaps going for the mouth may be the way to Attack Its Weak Point. If the attacking creature has a Breath Weapon this attack could cause a Superpower Meltdown. Eat Me can be a way to just bring the mouth to you so you can attack it. An Eat the Camera shot can be used for extra effect.
A Sister Trope to Eye Scream (another part of the face where an attacks are considered brutal), Groin Attack, Breast Attack, Literal Ass-Kicking, Agony of the Feet (all serious attacks on sensitive areas).
Not to be confused with Offing the Mouth (as "mouth" here refers to a person mocking another).
- While fighting an unstoppable monster called Chaos, The Savage Dragon knocked out its partner, a mutant with high-pressure dung hoses for Arm Cannons. He shoves the guy's arm into Chaos' mouth, cries out "Eat shit and die!", and activates the shit-slingers with enough force to make Chaos' head explode.
- A story in The Punisher has Frank dealing with a corrupt CIA agent in South America. Frank fits his fist down the man's throat and shows what happens when someone opens their fist inside a person's mouth.
- When Robin and Spoiler find themselves fighting a group of mooks lead by a man who is incredibly proud of his bizarre dental work Robin immediately kicks him in the mouth when he's close enough, causing him to loose a tooth.
- In How to Train Your Dragon, the Red Death is defeated by Toothless spitting a plasma shot in its mouth, blowing it up from the inside. This is foreshadowed when he sees a small dragon do a similar thing to another dragon when fighting for food (just that one was not a fatal blast).
- Beldar of Coneheads has been sentenced to fight for survival against the monstrous Grelblatt for "bonding" with inferior humans. Beldar uses his golf skills to launch one of the monster's turds into its mouth, where it lodges in its throat, asphyxiating it. This earns Beldar a royal pardon.
- Played for laughs in Dragonheart, where Sir Bowen stabs the last dragon into the roof of its mouth as a natural response to it trying to eat him. With him still inside the mouth, his leads to a Mexican Standoff where neither character can attack or withdraw safely. Eventually, after many hours of being stuck in this position, they agree on a truce and separate.
- In The Lord of the Rings: The Fellowship of the Ring, Legolas kills the cave troll by shooting an arrow through its mouth into its brain.
- In King Kong (2005), Kong rips out the tongue of one dinosaur he fights, and then kills it by breaking its jaws.
- In Godzilla (2014), the titular monster finishes off the female MUTO by prying her jaws open and unloading his atomic breath down her throat.
- In Reign of Fire Reluctant Hero Quinn learns about the weakness of dragons from feisty Denton. Dragons' fire-breathing is done by spraying a combustible fluid and an oxider from twin ducts in their mouths. Rupturing a dragon's palate results in the fluids mixing too soon, which makes his head explode. Quinn exploits this tactic to defeat the Final Boss dragon.
- At the end of The Rock, Stanely takes out the (true) villain of the movie by shoving a ball of the nerve gas in his mouth.
- A segment in Tales from the Darkside: The Movie, "Cat from Hell", ends similarly to the Stephen King short story it's based on, where a hitman is hired to kill a vengeful cat, but the cat instead kills the hitman by jumping into his mouth. Here it goes even further, as the cat waits all night inside the hitman's body until its real target Drogan, an old man who did animal testing (and hired the hitman), comes home and sees the cat crawl out from the hitman's mouth, causing Drogan to have a fatal heart attack at the sight.
- Deadpool: During the brawl inside the car at the beginning, after burning the forehead of a mook with the car's cigarette lighter, Deadpool forcefully puts it in his mouth.
Deadpool: I never say this... but don't swallow.
- In The Alice Network, Eve bites Renés mouth. (It leaves a scar.)
- In the Stephen King story The Cat from Hell, a hitman is hired to kill a cat (who is avenging an drug experiment which killed thousands of cats years before), but the cat takes the hitman out by leaping into his mouth, and causing the hitman to choke to death as the cat crawls all the way down his throat. The cat even manages to make sure the hitman's mouth was open by a Groin Attack with its claws, getting a yawning scream of pain from the hitman.
- Everworld: One of the Vikings is known as Sword-eater due to having taken a sword through both cheeks.
- Robert A. Heinlein's Glory Road. While Oscar Gordon and Star are fighting a dragon they fire several arrows into its mouth — one of the few places on a dragon's body where they can penetrate.
- In Tribesmen of Gor Tarl is part of a hunt for a legendary old shark. At one point the shark grabs the leader in his jaws, and while the shark's jaws are open Tarl stabs and slashes at it with a sword.
- Dungeons & Dragons 1st Edition Advanced D&D Player's Handbook. The Poison section mentions the technique of throwing a poison potion into a monster's mouth, hopefully killing it.
- Broken Age: Vella fights Mog Chothra by forcing open its mouth and shooting the death ray into it.
- Several of the giant mechanical sea creature bosses in Darius are only vulnerable in their mouth when it's open, such as King Fossil (plus many of its variations) and Fatty Glutton.
- The Dark Souls games mostly avert this, as the mouths are rarely the weak point.
- Dark Souls 1 even tries to trick the player into thinking this can be done to Chaos Eaters, who have their mouths on top of their heads. But leaping onto them doesn't do the usual Critical Hit, but instead gets the player chewed up.
- Dark Souls II does a variation. The Demon of Song is a giant frog, but what looks like its mouth is actually a cover for its face and extra set of arms. Yet that face within the mouth is the only vulnerable part of its body (unless the player uses poison cloud attacks).
- In Demon's Souls the Rock Worm takes a lot more damage when hit in the mouth (done either with spells, arrows, or a long melee weapon with a vertical swing).
- The God of War series:
- Several Action Commands attacks involve slamming the Blades of Chaos/Athena into a monster's mouth, and pulling until either the blades tear through the mouth (to do it again) or the monster's jaw is ripped open (the killing move).
- The finishing move against Minotaurs is to slam one of Kratos's blades down their throat.
- In The Legend of Zelda: Twilight Princess the Deku Toad is defeated by repeatedly attacking its tongue.
- In the Metroid series, this is used for several bosses, including the mainstays Kraid and Ridley.
- Resident Evil 4 does this for some enemies.
- The Del Lago (the giant beast lake salamander) takes extra damage if hit in the mouth instead of its body.
- Salder in the "Separate Ways" missions has to be hit until the eye in his mouth is exposed, and hitting that enough times will defeat him.
- The Mark Remark spoofed how getting punched in the mouth was treated as serious and wrong, despite such injuries being expected in professional wrestling.
- In The New Adventures of Superman, "The Robot of Riga", Superman flies into the mouth of the Humongous Mecha to sabotage it from the inside.
- The Venture Bros.: A particularly cringe-inducing scene from "The Incredible Mr. Brisby" has Brock pole-vault over a fence, using a live man's throat as a planting box.