In reference to a man's penis who has sex with the same girl that at least two of his friends have already been with. He is by definition removing dried jizz that was left behind from his friend's climax. This cleans the girl out for the rotation to continue.
Chris: I'm going to get some from Briana tonight! Hell yeah!!!
Kyle: Dude, I've already had that chick.
Dan: I just had her last night! It was bad enough I got Kyle's sloppy seconds.
Chris: I guess I get to be the cum chisel....fuck my life!
To have a guy bend over with his ass in the air and have another guy side straddle the other guy and push his scrotum in his rectum. The following looks like a wrench and creates torque on the bottom guy's rectum.
Bob: im a catcher.
Chester: if thats the case, lets do some wrenching!