In gay lingo, a pig is generally a bottom who is extremely interested in, and engages heavily and to excess (i.e. "wallows" in) a particular aspect of play or sexuality. It is frequently used as a descriptive and only mildly (if at all) derisive term toward someone with these strong interests. A comparative term in heterosexual kink would likely be "slut."
For instance, a cum pig has a great love and fascination for semen/ejaculate and often craves playing with it. A pain pig is a masochist who never seems to get enough pain play.
Not to be mistaken for piggy play, pig play, or the role pay pig.
