A dry david is the opposite of a wet willy. It is when you stick your dry finger and insert it into someone's ear and twist.
Zack just gave Marco a dry david.
A Dry David is basically the exact opposite of a WET WILLY. It is were you stick your finger in a victim's ear and stwist it around violently. But your finger is not wet so it hurts.
Last night I gave my teacher a Dry David.