The character who is always the butt of the demeaning joke or the "put him through hell" plotline. For whatever reason, the Butt Monkey seems to walk through life with a permanent "Kick Me" sign attached to their backs, invisible to them, but all too visible to the rest of the world. Nothing ever goes right for this character, and if something bad is going to happen to someone, chances are it's going to happen to them. Long story short, it sucks to be the Butt Monkey.
Simply having a character go through hell once or twice (no matter how severely) is not enough to be the Butt Monkey. With primary and secondary characters, it must be a regular occurrence. With tertiary characters, it must be their sole purpose to exist. They don't do anything to earn the hell they go through, other than being an easy target. Sometimes it's all the writers can think of to do with the character.
The direct opposite of a Karma Houdini (although technically the definition applies to those who escape from karma regardless of whether it's good or bad) If carried too far, may result in Deus Angst Machina. This can be counteracted if you occasionally Throw the Dog a Bone, though many writers just can't resist Yanking The Dog's Chain. The Butt Monkey is occasionally dangerous if he's pushed too far.
If the audience sympathizes with them, they become The Woobie. If, however, the character's suffering is Played for Laughs and the reason the audience loves them is because of all the crap they get put through, they're generally The Chew Toy. On the other hand, if the audience begins to resent the "unfair" treatment of the character, they can become the Designated Monkey. The joke can also be dragged too far in other ways - to make a character a Butt Monkey of fate is one thing; to have them constantly taking the punishment for the misdeeds of an unsympathetic cast is another.
If a female Butt Monkey is regularly used for slapstick and it is Played for Laughs she is probably also a case of Slapstick Knows No Gender. If every conceivable misfortune happens to the Butt Monkey, regardless of its logic or lack thereof, you've got yourself a Cosmic Plaything.
If someone is Giving the Sword to A Noob, nine times out of ten this person is the noob. On the other hand, if someone in the cast other than The Hero is going to Take a Level in Badass, its almost invariably the Butt Monkey who does it.
In Yiddish, this character type is called a Schlimazel.
No real life examples, please; we'd be here all day.