Duck's Dealer

Listen up, dummy. The joke is playing on the word "quack." A duck makes a "quack" sound, but "quack" is also slang for a bad doctor. "Quack dealer" is also slang for drug dealer. You are so dumb that you thought the joke was about ducks crossing the road. I bet you think ducks are smarter than you. Also, your brain is smaller than a duck's brain.