Literally, a person who performs
fellatio. Of course, the main usage is
idiomatic: basically, someone who is
unpleasant,
distasteful, or has
insulted or wronged you. One of
George Carlin's
Seven Words you can Never Say on Television.
The word is quite a serious insult. Usually.
One of my former roommates and his friend would often, and loudly, address each other as "cocksucker", " motherfucker", and such. They did this playfully, almost affectionately. It must have been one of those slapping each other on the back macho guy camaraderie things.