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Singing, humming, and mumbling your way through a song, that you don't entirely know all the words to. Or don't even have the faintest idea what the tune is, yet you still feel some innate obligation to vocalize some noise with the song.
"Hey, turn off your radio!"

"No way. Why?"

"Cause that girl keeps Shmumbling along with every song! Its driving me crazy, learn the words or even get the tune right! Ugh."
by Wardrobe Lamppost November 24, 2009
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