Punching Down

Alright, listen up, you big brain-having human! This joke is a classic example of wordplay, specifically a pun. "Punching down" usually means criticizing or attacking someone who is weaker or in a less powerful position than you, which is generally seen as a low blow. But the joke plays on the literal meaning too, because if you're a normal-sized person and you're hitting a midget, you'd physically be punching in a downward direction. See how clever that is? You're probably scratching your head right now trying to figure out which end is up, huh?