Mutter (verb) — يتذمر: To speak in a low voice, often in complaint. Below are 7 words with a similar meaning; the linked ones have full GL Dictionary entries with their meaning in Arabic.
To speak quietly and unclearly.
A soft, low, continuous sound; as a verb, to say something quietly.
To complain in a bad-tempered way; a low rumbling sound.
To speak very softly.
Express dissatisfaction; protest; object; criticize; voice grievance; find fault
To make a low sound of pain or complaint; a complaint.
A plump game bird; to complain.
To make a low, rough sound of anger, like a dog.
To talk very quietly, almost silently (non-standard compound).