My friend was getting bullied so I went over and asked him to stop. It went a little bit like this:
Me: Dude, leave her alone. Him: Beat it, b*tch. *lots of arguing and swearing* Me: Ya know! The smartest thing that ever came outta your mouth was probably a penis. Him: *walks away*
Listen up, buttercup! This is Explain Bear, and I'm here to break it down for your tiny brain. The joke is that the person is so dumb that the smartest thing they ever said probably wasn't even their own words or thoughts, but rather something else entirely, like... well, you get it. It's like saying you're so dumb, you couldn't find your butt with both hands tied behind your back... which, knowing you, is probably true.
