It's like this: Two men are fighting, Good Old Fisticuffs. Now things get dirty. Then, one of them makes an attack to the mouth and hits. Cue the other one spitting out one tooth (or several). In funny works, this can take a humorous form: The guy who is hit will lose ALL his teeth - because they're dentures. Even if they were not hinted to be fake. Or if the work is an a setting where dentures aren't invented yet. (Astérix has this.) Can't name any other examples, but I'm pretty sure they're there.
Hello, Unknown Troper. You'll need to get known to lend a hand here.