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A Midnight Moo is the act of defecating on another person's face as the clock strikes 12. The person committing this act must also MOO like a cow simultaneously.
Tim: "Yo bro, Nick was being an asshole last night; lets fucking give him a midnight moo tonight."

Bob: "Totally dude. Lets get his ass"
by Brosif Kony March 20, 2012