A way of saying "wetback" without infuriating all the mexicans in the general area.
Guy 1: Hey look at all those moistbacks working in the back of that KFC
Guy 2: HAHA how pathetic!
A second or third generation mexican. Someone who was dried up but is still a dirty frijolero nonetheless.
Joe: I heard your daughter got knocked up by a wetback.
Steve: No, his parents were the wetbacks, he's a moistback.
Joe: So this one doesn't pick tomatoes?
Steve: No, he does drive-by's.