In Math, the complement is, simply put, that which is outside of the defined region. However, this is often used as an insult in geek circles.
Geek 1:Your momma's so fat, that she's the complement of your house.
Geek 2: Yeah, well at least my penis isn't always the complement of a vagina.
Geek 1: *shoots self, thus becoming the complement of existing*