Past tense of 'grouse'; complained or grumbled about something.
From 'grouse' as a verb (possibly from Old French 'groucier' meaning to grumble, or imitative in origin), plus past tense '-ed'. The verb form emerged from the noun for the bird, perhaps through association with the bird's harsh calls.
Soldiers in World War I groused constantly in their letters home—the word became so associated with military complaints that it's still used to describe that particular kind of resigned griping about unavoidable situations.
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