hum

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hum /ˈhʌm/ verb
hums; hummed; humming
hum
/ˈhʌm/
verb
hums; hummed; humming
Learner's definition of HUM
[no object] : to make a low continuous sound
: to sing the notes of a song while keeping your lips closed
[no object]
[+ object]
[no object] : to be very active or busy

— hum

noun [singular]