To complain in a low voice or express dissatisfaction in a muttering way, often about minor annoyances.
From Middle Dutch 'grommelen' or Germanic roots, possibly imitative in origin, meant to mimic the sound of low complaints. The word carries the sense of growling like an animal.
The word sounds like what it means—say it out loud and you're already grumbling! It's onomatopoeia, imitating that growly, discontented sound people make when they're annoyed.
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