In a grumpy manner; in a bad-tempered, sulky, or irritable way.
From grumpy with the adverbial suffix -ly added. The -ly suffix for forming adverbs comes from Old English -lice, which is related to 'like,' creating adverbs meaning 'in the manner of.'
Every time we add -ly to an adjective, we're using a 1,200-year-old English suffix that originally meant 'like'—'grumpily' literally means 'in a grump-like manner,' preserving ancient word-formation logic!
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