Most soft, fluffy, or covered with down feathers; having the greatest downy quality.
'Downiest' is the superlative form of 'downy,' using the '-est' suffix (Old English 'est') to indicate the highest degree of the quality described.
The superlative forms in English ('best,' 'worst,' 'downiest') are remnants of inflectional languages—we still mark extremes with special endings even though we've lost most other grammatical endings!
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