Merriam-Webster Collegiate Dictionary - hum

 I. verb  (~med; ~ming)  Etymology: Middle English ~men; akin to Middle High German ~men to ~, Middle Dutch hommel bumblebee  Date: 14th century  intransitive verb  1.  a. to utter a sound like that of the speech sound m prolonged  b. to make the natural noise of an insect in motion or a similar sound ; drone  c. to give forth a low continuous blend of sound  2.  a. to be busily active the museum ~med with visitors  b. to run smoothly the business started to ~  transitive verb  1. to sing with the lips closed and without articulation  2. to affect or express by ~ming ~med his displeasure  • ~ noun  • ~mable adjective  II. chiefly British variant of hem