Her: Eat my ass!
Me: Yes, chef!
Her: Eat my ass!
Me: Yes, chef!
Listen up, buttercup. This joke is funny because the dude is being asked to do a nasty thing. But instead of being grossed out, he answers like he's in a kitchen, like a chef getting an order. It's unexpected and kinda silly, you know? Since you didn't know, I bet you still eat with your hands.