The act of bending over whilst nude so that the "victim" gets a view of both the anus and the dangling scrotum. It is quite common to have both hands placed on the butt cheeks which are then used to pull the ass further apart, thus giving the viewer a better look at the anus.
Guy #1: "Hey man, what did you get your mother for Mother's Day?"
Guy #2: "I was all out of money so I just gave her a fenks."
James fenksed Sally.
Troy thought it was the middle of the night, turns out he was just looking straight into the anus of an almighty fenks.