A buttgig, is when somebody has had so much anal sex, it's almost as if the amount of people that would attend a rock concert, have been inside the person's arse.
Richard: wow, she loves anal!
Alex: But is it like a buttgig in there?
or
Nick: Man, he fucking loves getting bummed Isaac: Serious buttgig going onright there.
An insult, typically used when angry. The word, despite the word 'nigger', has no racist connotation to it, and can be used when insulting a member of any race.