The more litteral sense of cumming your pants.
Nobody cums their pants unless they are on a Tokyo Subway but you would totally preem yourself over the girl across the road.
Paul: Yeah, he's preem'n.
Johnny: ay homie, check out these new kicks i copped at the sto(re)...
Big Syke: yoooo son, them shits is preems
D-Boy: Anybody lookin' to grab? You know I always got that preems chronic, dat piff...
Fiend: worrrddd, yo can you front me a quarter homie? I don't got the scrilla right now mang..
D-Boy: you best get your broke ass up out my face homeboi, this chi-town's finest, and i ain't goin' out like no sucka