v. & n.
v. 1 intr. speak low in a barely audible manner. 2 intr. (often foll. by against, at) murmur or grumble about. 3 tr. utter (words etc.) in a low tone. 4 tr. say in secret.
n. 1 muttered words or sounds. 2 muttering. mutterer n. mutteringly adv.
[ ME, rel. to MUTE ]