Complaining or expressing discontent in a low, quiet voice; dissatisfied muttering or complaints.
From Middle English 'grumble,' possibly from Dutch 'grommelen' or an imitative origin that mimics the sound of low rumbling complaints. The word first appeared in English around the 1500s.
Grumbling is technically complaining, but the way the word SOUNDS—all those rumbling 'gr' and 'mb' sounds—actually mimics the low, unhappy voice of someone muttering. English is full of these onomatopoetic words where the sound matches the meaning!
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