Inhumane

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late Middle English (in the sense ‘inhuman, brutal’): originally a variant of inhuman (rare after 1700); in modern use from in-1 ‘not’ + humane (the current sense dating from the early 19th century).

wiktionary

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Originally a variant spelling and pronunciation of inhuman, ultimately from Latin inhūmānus; it appears to have died out 17c. but returned c. 1822, probably a reformation as a negative of humane. [1]

etymonline

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inhumane (adj.)

originally a variant spelling and pronunciation of inhuman "cruel, hard-hearted;" it appears to have died out 17c. but returned c. 1822, probably a reformation as a negative of humane (q.v.), with its accent.