unjust (adj.)

prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.

It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.

late 14c., "morally upright, righteous  in the eyes of God" ("Now chiefly as a Biblical archaism" - OED); also "equitable, fair, impartial in one's dealings;" also "fitting, proper, conforming to standards or rules;" also "justifiable, reasonable;" from Old French juste "just, righteous; sincere" (12c.) and directly from Latin iustus "upright, righteous, equitable; in accordance with law, lawful; true, proper; perfect, complete" (source also of Spanish and Portuguese justo, Italian giusto), from ius "a right," especially "legal right, law" (see jurist; from Latin ius also come English jury (n.), injury, etc.).

From c. 1400 as "right-minded, good in intention;" from early 15c. as "legal, lawful, right in law." Also "exact, precise; marked or characterized by precision; having correct dimensions" (late 14c.); of narrations, calculations, etc., "accurate, correct" (early 15c.). The sense in music, "harmonically pure, correct, and exact" is by 1850.

The more mundane Latin law-word lex covered specific laws as opposed to the body of laws. The noun meaning "righteous person or persons; Christ" is from late 14c. (The neuter adjective in Latin was used as a noun, iustum, "what is right or just").