also: もにゃもにゃ
adverb, adverb taking the 'to' particle
mumbling; muttering; incomprehensibly
Mimetic word describing the sound or manner of speaking unclearly, often in one's sleep or when talking to oneself. Frequently used with 言う or 呟く.
寝言でむにゃむにゃ言っている。
He's mumbling something in his sleep.
彼はむにゃむにゃと何
He muttered something incomprehensibly.
ぶつぶつ implies grumbling or complaining in a low voice, often with discontent, while むにゃむにゃ focuses on the unclear, muffled quality of speech, often without negative emotion.
もごもご also describes mumbling, but often with the nuance of speaking with one's mouth full or moving the mouth indistinctly, whereas むにゃむにゃ emphasizes the sound itself.
Onomatopoeic word imitating the sound of indistinct, sleepy, or muffled speech. The exact origin is uncertain, but it is a typical reduplicative mimetic formation.