undue 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.

mid-14c., "customary, regular, right, proper;" late 14c., "owed, payable as an obligation, owing by right of circumstance or condition," from Old French deu, past participle of devoir "to owe," from Latin debere "to owe," originally, "keep something away from someone," from de- "away" (see de-) + habere "to have" (from PIE root *ghabh- "to give or receive").

Of actions, "conscientious, careful," late 14c. Meaning "that is to be expected or looked for" is by 1833. Phrase in due time "at a set time; at an appropriate time" is from late 14c. Due to is from early 15c. as "deserved by, merited by;" also "owing to." It is attested from 1660s as "attributable to as a cause or origin." Its use as a prepositional phrase (much maligned by grammarians) is by 1897.

late 14c., "without due moderation; improperly, unsuitably;" see undue + -ly (2). From early 15c. as "unjustly, wrongfully."