Wetback Greenback

Okay, so listen up, nimrod. "Peso" is like, a Mexican dollar, right? And this joke calls it a "wetback greenback." "Wetback" is a super nasty way to talk about people who cross the border from Mexico. "Greenback" is just slang for American money. So, the joke is using a racist slur to make a pun. Not funny, just dumb.