Someone who has anal sex with their partner.
Generally considered to refer to where the man's penis will enter the rectum, from behind the other person, in that both parties are usually facing the same general direction.
This meaning, that the back of the one, if facing the front of the other.
Refers to the rectum, the anus of a person.
Can refer to gay sex, specifically anal penetration by another person, or by assorted sex toys.
Where a man will enter a woman from behind, either anally or vaginal sex.
Noteworthy: Popular use within gay adult videos and site descriptions.