1. To take a shit and pee.
2. A reference to blackjack; a situation where you have to give up a good hand for something else. In other words, being forced into a situation where you give up something good for something that could suck.
3. You have 21 Legos in your hand and you decide this needs to end.
"Yo, I gotta drop 21."
"I JUST picked up the last piece of chocolate cake, but then my mom made me drop 21 and give it to my brother because he hadn't had any yet. Lame!!"