Superlative form of 'hasty': most inclined to rush or act without enough thought; moving with the greatest urgency or speed.
From 'hasty' + superlative suffix '-est'. The superlative '-est' comes from Old English and Germanic languages and marks the highest degree of comparison.
Interestingly, we can say 'the hastiest person' but 'more hasty' sounds better than 'hastier' to many speakers, showing that adjective conjugation patterns are shifting in modern English as we favor 'more/most' over '-er/-est'.
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