means To speak in a low, soft, indistinct voice, or to make a continuous hushed sound — and, by extension, to grumble a quiet complaint.

from From Latin 'murmur,' meaning a humming or muttering — itself an echoic word, born from the very sound it names, like a brook or a low grumble doubled back on itself. Many languages reach for the same soft 'm' and 'r' sounds for this idea, possibly because that's simply what quiet, indistinct speech feels like in the mouth. It drifted into English through Old French 'murmure.'