Speaking in a low, unclear voice, usually while complaining or thinking aloud.
From Middle English 'muteren,' of imitative origin, mimicking the sound of indistinct speech. The '-ing' form indicates ongoing action.
Muttering is universal across languages because it's literally trying to copy the sound itself—'mutt-mutt' mimics the noise of unclear grumbling, which is why you hear similar words in German 'maulen' and Dutch 'mutte.'
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