Acronym derived from the Old English Law defining an illegal sexual act as "For Unlawful Carnal Knowledge". Mainly used by the lower class English in the beginning, it's use has spread globally over the centuries.
When you drive your huge hard cock into your girlfriend's pussy, to give her the "big o" with your big one.
There are a number of slang terms for fuck. Among the most common are sex, sexual intercourse, make love, and go all the way.
A dumb, stupid, idiotic, ugly, bizarre, annoying, trifling person. Can be used to describe a troll, or someone who does not give a shit about life or anyone else.