undefeated 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., defeten, diffaiten, "overcome (with sorrow or anger)," from Anglo-French defeter, from Old French desfait, past participle of desfaire "to undo," from Vulgar Latin *diffacere "undo, destroy," from Latin dis- "un-, not" (see dis-) + facere "to do, perform," from PIE root *dhe- "to set, put."

From early 15c. as "bring ruination, cause destruction" (now obsolete in this sense); from late 15c. as "frustrate, prevent the success of." Sense of "deprive of something expected, desired or striven for" is from 1530s. Meaning "overcome in a contest of any kind" is from 1560s. Related: Defeated; defeating. Compare defect, deficient.