means To make a low, continuous, droning sound — either with closed lips while not quite singing, or as the steady noise of machinery, bees, or busy activity.

from An imitative word — it sounds like the thing it names, built from the soft mmm a closed mouth makes. English has had 'hum' since the medieval period, alongside echoic cousins like 'hem' and 'haw,' all born from the noises people make when humming, hesitating, or mumbling. The bee-buzz sense and the 'place is humming with activity' sense both grew naturally from that same droning hum.