You're so skinny when you lift up weights, you fall through your asshole.
Skinny Shaming
Explanation
Explain Bear
Alright listen up, buttercup. This joke is saying you are so damn skinny, that if you tried to lift weights, you'd be so light that gravity would just yeet you right through your own butthole. Which, let's be honest, is pretty sad. You probably couldn't even bench press a cheeseburger, you twig.