Hey, you down to fuck?
No, I’m just down.
Hey, you down to fuck?
No, I’m just down.
Alright, listen up, genius. This joke plays on the double meaning of the word "down." The first person asks if the other person is "down" as in "willing" to do something. The second person says they are just "down" as in someone with Down syndrome. Get it? Bet you didn't!