Superlative form of dandy; the most fashionable, fine, or characteristic of a dandy's excessive attention to appearance.
From 'dandy' plus the superlative suffix '-est' (from Old English), indicating the highest degree of the quality.
Using 'dandy' as an adjective meaning 'fine or excellent' is actually older than using it to mean 'a vain person'—it shows how the same word can develop opposite meanings through different paths!
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