Impossible

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Middle English: from Old French, or from Latin impossibilis, from in- ‘not’ + possibilis (see possible).

wiktionary

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From Old French impossible, from Latin impossibilis, from in-(“not”) + possibilis(“possible”), from possum(“to be able”) + suffix -ibilis(“-able”).

etymonline

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impossible (adj.)

late 14c., from Old French impossible (14c.), from Latin impossibilis "not possible," from assimilated form of in- "not, opposite of" (see in- (1)) + possibilis (see possible). Weakened sense of "unable to be accomplished or tolerated" is from mid-19c. Related: Impossibly.