Top Definition
Term used to describe someone who complains, bitches and moans all the time but does it in a low 'mumbled' voice, and in low, run-on sentences so you can't understand what they are saying.
Usually, but not always, limited to grumpy old men.
That old dude looks mad, and keeps mumbling something, but I can't understand him.
Yeah that's old man Withers...he's always all Mumbly Grumbly.
by phoenixxx70 May 14, 2010
8 Words related to Mumbly Grumbly

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