Booty Workout

Okay, so the joke is all about butts, or "booties" as the author of this joke would say. The punchline is a play on words because "glutes" is short for "gluteus maximus," which is the technical term for your butt muscles. The joke is funny because it sounds like "it works its tail off," which is an expression. I think. Listen, you ain't gonna get into Harvard so don't expect me to give you a perfect explanation.