1650s, "not consistent with moral law or standards, ethically wrong," from assimilated form of in- (1) "not" + moral (adj.). In legal language it tends to mean merely "contrary to common good or reasonable order." Related: Immorally.
word-forming element meaning "not, opposite of, without" (also im-, il-, ir- by assimilation of -n- with following consonant, a tendency which began in later Latin), from Latin in- "not," cognate with Greek an-, Old English un-, all from PIE root *ne- "not."
In Old French and Middle English often en-, but most of these forms have not survived in Modern English, and the few that do (enemy, for instance) no longer are felt as negative. The rule of thumb in English has been to use in- with obviously Latin elements, un- with native or nativized ones.
mid-14c., "associated with or characterized by right behavior," also "associated with or concerning conduct or moral principles" (good or bad), from Old French moral (14c.) and directly from Latin moralis "proper behavior of a person in society," literally "pertaining to manners," coined by Cicero ("De Fato," II.i) to translate Greek ethikos (see ethics) from Latin mos (genitive moris) "one's disposition," in plural, "mores, customs, manners, morals," a word of uncertain origin. Perhaps sharing a PIE root with English mood (n.1).
From late 14c. as "of or pertaining to rules of right conduct" (opposed to non-moral, amoral) and "morally good, in accordance with rules of right conduct" (opposed to immoral). Of persons, "habitually conforming to moral rules," 1630s. From 1680s with reference to rights, duties, etc., "founded on morality" (opposed to legal).
Applied to indirect effect in moral support (1823), moral victory (1888), where the notion is "pertaining to or affecting the character or conduct" (as distinguished from the intellectual or physical nature), a sense attested from 1590s; in this sense, compare morale. Related: Morally.