in an elfish manner; in a mischievous, playful, or elf-like way.
From elfish (adjective) plus -ly (adverb suffix). The -ly suffix converts adjectives into adverbs describing manner or quality.
Describing someone as acting elfishly suggests they're being playfully mischievous—not mean-spirited, but delightfully tricky in the way folklore elves have been portrayed for centuries.
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