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This occurs when someone only has a vague recollection of a song, and is attempting to sing it, but cannot remember all of the lyrics. As the person attempts to sing the song, they alternate humming, when they have no clue about the lyrics, with mumbling, when they think they can get away with the words they remember. The result is highly obnoxious.
Person 1: The hmm-mmm-mm don't like it, rock the hmm-mmmm, rock the hmmm-mmm.

Person 2: Dude, stop hummumbling the damn thing and put on the mp3.
by Matthew F.. May 21, 2008
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