according to world english dictionary shaft is a slang term meaning "to have sexual intercourse with" and Sack as in a Ballsack a.k.a Your Testicles inside your Scrotum, so Shaft My Sack Means "To Be Intimate/Sexual With Ones Testicles"
Your Doing Real Good In COD So At The Lobby You Say to Them Shaft My Sack
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.