To emit a continuous low droning sound like that of the speech sound (m) when prolonged.
To emit the continuous droning sound of a bee on the wing; buzz.
To give forth a low continuous drone blended of many sounds: The avenue hummed with traffic.
To be in a state of busy activity.
To produce a tune without opening the lips or forming words.
To sing (a tune) without opening the lips or forming words.
Baseball To throw or pitch (a ball) very fast.
The sound produced by humming.
The act of humming.
[huhm-uh-buh
l] 
(hŭm)