Improbable

google

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late 16th century: from French, or from Latin improbabilis ‘hard to prove’, from in- ‘not’ + probabilis (see probable).

wiktionary

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Borrowed from Middle French improbable, from im- +‎ probable.

etymonline

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improbable (adj.)

1590s, from assimilated form of in- (1) "not, opposite of" + probable, or else from Latin improbabilis. Related: Improbably.