'shit or git' means get going and do it, otherwise leave. Don't waste everyone's time. Expression essentially comes from 'sit on the toilet and do 'it' (shit), otherwise stop wasting time. Leave and let someone else use it.'

To clarify, "git" is slang from the southern USA, and means "leave." I think it is a shortening of "git going" (i.e., "get going") PS - I want to disagree with part of what Whitney said. "What goes around, cones around" means that when you do something bad, often you end up being harmed later. I dont think it relates to shitting or gitting

I would interpret this as " what goes around comes around and it's now or never" like, 'it's your time to do what you came here to do' or ' you gotta do what you gotta do'. Something like that! Hope that helps! Enjoy reading!