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1.
To royally screw something up.

See Epic Failure
Boy! He really DeMelloed that one, didn't he?
by JustUsChickens2 August 01, 2008
 
2.
{v} One who did something completely stupid and dumb. A dumb action. To act in a way that makes one appear simple. To use absolutely no common sense. To ask a question which has clearly been previously answered.
Person A: I zipped up my pants and caught my ball sac in the zipper.
Person B: Dude, you just Demelloed!

Guy 1: So I learned how to do burnouts last week! Its so cool!
Guy 2: Nice man! I bet you get the pussy with that!
Guy 1: Yea dude, it's sweet....but I can't figure out why my car is starting to break.
Guy 3 (to Guy 4): Dude, he just demelloed!
by one88888 August 05, 2008