Most covered with hair or having the most hair; the superlative form of hairy.
From Old English 'hær' (hair) + '-y' (adjective suffix) + '-est' (superlative suffix, from Proto-Germanic 'istiz').
The superlative '-est' is one of the oldest word-building tools in English, dating back thousands of years to Proto-Indo-European languages, which is why we still use 'hairiest' instead of something like 'most hairy' for short, simple adjectives.
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