when one of your sick little friends lets off a bad one or you just happen to be walking through a chemical waste depository
Dude: man, that hums like your mums cunt
Smelly Hobo: hey guys
Guy: dude, you hum
An alternative to the traditional hum is that of 'Huh?.....mmm'
Occasionally the word may be used in the adjective to described an often hummed individual or for an event that recieves many hums.