Injustice

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late Middle English: from Old French, from Latin injustitia, from in- ‘not’ + justus ‘just, right’.

wiktionary

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From Middle English injustice, from Old French injustice, from Latin iniustitia. Equivalent to in- +‎ justice.

etymonline

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injustice (n.)

late 14c., from Old French injustice "unfairness, injustice" (14c.), from Latin iniustitia "unfairness, injustice," from iniustus "unjust, wrongful, unreasonable, improper, oppressive," from in- "not" (see in- (1)) + iustus "just" (see just (adj.)). Injust (adj.) is attested from late 15c., from French, but unjust is the usual English word.