To run fast.
The cops showed up so I had to shag ass.
v. to flee from an area, typically to evade trouble
Guy 1: "Yo man, you got the goods?"
Guy 2: Hell yeah, give me just on sec my man."
Guy 1: "Oh shit! It's the fuzz! Lets shag ass!"
Guy 1: "Dayyyummm! Look at that fine bitch comin'this way."
Guy 2: Nigga, dats my ex! The one I was tellin' you bout who won't stop callin' my house. Lets shag ass!"
the act of shaging someone's ass
If you shag ass her, then shed be waddling for days.