An insult, typically used when angry. The word, despite the word 'nigger', has no racist connotation to it, and can be used when insulting a member of any race.
That buttnigger just cut me off!
No no, doesn't mean you have shit all over your ass. Just an insult, that I THOUGHT I made up, but it looks like its already here! >:O
Fat Girl: If I eat your sandwhich, will you get angry at me?
Starving Man: Uh, why you ask?
Fat Girl: I WAS HUNGRY!
Starving Man: FUCKING BUTTNIGGER!