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.
"Fucking the shit out of girl" means you fuck her so good she talks about it to her friends.
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"Fucking the shit out of" can also mean fucking her hard and rough. It's open for interpretation. But common sense notion is: SEX WAS FUCKING AWESOME (for her)!