Superlative form of fawny; most fawn-colored or most resembling a young deer in appearance or behavior.
From 'fawny' plus the superlative suffix '-est,' creating the highest degree of the quality. This is one of English's two methods for forming superlatives (alongside 'most' for longer adjectives).
English adjectives can form comparatives two ways—'fawnier/fawniest' or 'more fawny/most fawny'—and linguists still can't perfectly predict which way a word will go, which means even native speakers sometimes have to choose between sounding natural and sounding correct.
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