Stands for "suck my pussy," derived from one of Sophitia's special grab move in Soul Calibur III, where she takes her opponent's head, puts it between her legs and jumps on the ground jamming her crotch in their face at high velocity. First stated by an intense brown gamer when he was losing in frustration to a stupid white guy, where he rejoiced "suck my pussy" when he won.
Now, SMP is used as the short form, synonymous to Feel It.
Want to face me in Soul Calibur? Get ready to Suck My Pussy!
While I was wrestling Michael, I SMPed that bitch.