Hum  —v. (-mm-) 1 make a low steady continuous sound like a bee. 2 sing with closed lips. 3 utter a slight inarticulate sound. 4 colloq. Be active (really made things hum). 5 colloq. Smell unpleasantly. —n. 1 humming sound. 2 colloq. Bad smell.  hum and haw (or ha) hesitate; be indecisive. [imitative]