A Character in a television show, manga, anime, comic book, webcomic, etc. who constantly gets abused and never gets ahead in life. Played for comedy.

A phrase to describe someone who is cognitively degenerating. Synonym of "going off the deep end". Can have varying degrees of severity.

Reference to the song "Hey You" by Pink Floyd. The line "and the worms ate into his brain" makes no sense in an otherwise linear and literal narration throughout the lyrics.

Rectum dwelling primate

Someone who has an invisible, permanent "Kick Me" sign taped to their back.
They're the butt of almost any demeaning joke, they commonly have horrible luck, and a large amount of horrible things happen to them. Their existence is just to solely have horrible things happen to them. If anything positive happens to them, the person usually gets screwed over in the end.

one who, through ineptitude, lack of confidence, lack of superpowers, or sheer ability to kiss ass, is the bitch of everyone around them

One who clings to the butt of another as a monkey clings to a tree, a butt-kisser.

A Butt-Monkey is someone who worships another person. They laugh at all their jokes whether they're funny or not, they think they're great even when they're not and generally strive to spend as much time with that person as they can. The person the Butt-Monkeys follow is called a Butt-Monkey King or Queen.