To participate in an activity that results in ejaculation. Usually exclaimed, directed at a person. Said person is most likely in a situation that the commentator believes will end in orgasm. "Nut" refers to semen, and thus, to get your nut, is to ejaculate.
When the female counterpart is aware of the meaning of this phrase, it is usually more polite to instead say, "GYN". The female may or may not understand this abbreviation.
Bill Gates is seen walking alongside a very attractive woman.
Super Mario: "Yeah Bill Gates, get your nut!!!!"
Luigi catches a glimpse of Mario walking into a secluded area of his back yard with an extremely attractive girl.
Luigi: "MARIO! YEAH DUDE!! GET YOUR NUT"
An expression guys tell to other guys that relates to getting inside or seducing a female.
In this scenario, the nutshell is the female, and inside the nutshell lies the nut. As gentlemen, it is our duty to get the nut.
Guy 1: Dude, are you going out with that girl tonight?
Guy 2: Yeah, bro
Guy 1: Get your nut!