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U Dum is a game where you, the player, tries to get the attention of another player in a variety of ways. After gaining their attention, you immediately shout "U Dum!" However, the defending player may preemptively shout "U Dum" after you have gotten their attention, nullifying your initial attempt.
Todd: "Hey Blair, I learned something interesting about you this weekend."

Blair: "Really? What's that?"

Todd: "I learned that U Dum!"
by weenius September 09, 2010
Perhaps the best singer in the world, Udum has won every emmy and oscar there is.
Udum rocks!!!!!! I hope he is reading this!!!!!!
by D November 21, 2004