(Verb): The act of having sex.
This is supported by the popular song "Freaks Of The Industry" sung by Digital Underground. The latter half of the chorus goes "...and when you see us backstage, be prepared to G". I believe it is a metaphoric allusion to the Grafenberg spot (also known as the G-spot
) which of course, refers to the theoretically ultra-sensitive area of pleasure inside a woman's vagina.