A slang phrase meaning, "If you don't hurry up and do something, you are gonna miss out." or "If you don't do this, you are gonna feel like a dumb shit (like you've been 'shitted on')and regret it later."
Note: 'before' can also be shortened to 'fo' for a more urban slang feel.
Tom: Man, I wanna go get that O of kind bud dude accross town was gonna sell me for half price, but I'm so lazy I don't feel like driving.
Greg: Dude, you better get the fuck on before you're shit the fuck on! Are you fucking retarded?