Merriam-Webster's Collegiate® Dictionary, 11th Edition
HUM
I. verb (hummed; humming)
Etymology: Middle English hummen; akin to Middle High German hummen to hum, Middle Dutch hommel bumblebee
Date: 14th century
intransitive verb
1. to utter a sound like that of the speech sound \\m\\ prolonged, to make the natural noise of an insect in motion or a similar sound ; drone , to give forth a low continuous blend of sound,
2. to be busily active , to run smoothly , transitive verb to sing with the lips closed and without articulation, to affect or express by humming , hum noun hummable adjective see: hem