Irresistible

google

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late 16th century: from medieval Latin irresistibilis, from in- ‘not’ + resistibilis (from resistere ‘resist’).

wiktionary

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ir- +‎  resistible

etymonline

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irresistible (adj.)

1590s, from Late Latin irresistibilis, from assimilated form of in- "not, opposite of" (see in- (1)) + resistere (see resist). Related: Irresistibly; irresistibility.