Origin and history of untruth

Middle English treuth, truþ, from Old English triewð (West Saxon), treowð (Mercian) "faith, faithfulness; fidelity to country, kin, friends; loyalty; disposition to be faithful; veracity, quality of being true; pledge, covenant."

This is reconstructed to be from a Germanic abstract noun from Proto-Germanic *treuwaz "having or characterized by good faith." This in turn is reconstructed in Watkins to be from PIE *drew-o-, a suffixed form of the root *deru- "be firm, solid, steadfast." With Germanic abstract noun suffix *-itho (see -th (2)). Compare troth, truce, trust (n.), tree (n.). English and most other IE languages do not have a primary verb for "speak the truth," as a contrast to lie (v.).

The sense of "something that is true, a true statement or proposition" is recorded by mid-14c. The meaning "accuracy, correctness, conformity of thought with fact" is from 1560s. It is attested by late 14c. as "that which is righteous or in accordance with divine standard; true religious doctrine; virtuous conduct." Truth! as an expression of assent or emphasis is by 1530s.

Let [Truth] and Falsehood grapple; who ever knew Truth put to the worse, in a free and open encounter. [Milton, "Areopagitica," 1644]

Truth squad in the U.S. political sense is attested in the 1952 U.S. presidential election campaign.

At midweek the Republican campaign was bolstered by an innovation—the "truth squad" ..., a team of senators who trailed whistle-stopping Harry Truman to field what they denounced as his wild pitches. [Life magazine, Oct. 13, 1952]

Truth-serum "injected truth-drug" is by 1925.

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."

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. 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.

Often euphemistic (untruth for "a lie") or emphatic, if there is a sense already of divestment or releasing: unpeel " to peel;" unpick "pick (a lock) with burglars' tools;" unloose for "to loosen."

It also makes words from phrases, such as uncalled-for, c. 1600; undreamed-of, 1630s. Fuller (1661) has unbooklearned. A mid-15c. description of a legal will has unawaydoable; Ben Jonson has un-in-one-breath-utterable. The word uncome-at-able is attested by 1690s in Congreve, frowned at by Samuel Johnson in the 18th century and by Fowler in the 20th ("The word had doubtless, two or three centuries ago, a jolly daredevil hang-the-grammarians air about it ; that has long evaporated ; it serves no purpose that inaccessible does not ....").

But the practice continued; unlawlearned (Bentham, 1810), unlayholdable (1860); unputdownable, of a book, is by 1947; unpindownable, by 1966. Also compare put-up-able-with (1812). As a prefix in telegraphese, to replace not and save the cost of a word, it is attested by 1936.

With the variety of its possible use, and the need for negatives, the number of un- words that might be made in English is almost endless, and that some are used and some never is owing to the caprice of authors.

Dictionary editors noted this since 18c. but also padded the list. John Ash's "New and Complete Dictionary of the English Language" (1775) has many pages of one-line un- entries; among a dozen consecutive entries are unhaggled, unhaired, unhalooed, unhaltering (adj.), unhaltering (n.), which sorts of words OED (1989) notes were "obviously manufactured for the purpose" and some turn up in other texts only decades later, if at all. (Ash vindicated.)