To make a low, continuous sound like that of a bee or a machine. The refrigerator began to hum softly in the background.
To sing with closed lips, producing a tune without opening the mouth. She hummed her favorite song while she worked.
To be in a state of busy activity. The office was humming with activity as the deadline approached.
(informal, chiefly British) To emit a strong, unpleasant smell; to reek. The locker room really hums after practice.