The superlative form of abrupt, meaning the most sudden or unexpected of all.
From Latin 'abruptus' with English superlative suffix '-est', following the same pattern as 'abrupter' but marking the extreme degree.
Superlatives like 'abruptest' help us express extremes, but in conversation most people just say 'most abrupt'—which shows how English is shifting toward simpler comparative forms.
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