Origin and history of half-truth

Old English half, halb (Mercian), healf (W. Saxon) "side, part," not necessarily of equal division (original sense preserved in behalf), from Proto-Germanic *halba- "something divided" (source also of Old Saxon halba, Old Norse halfr, Old Frisian, Middle Dutch half, German halb, Gothic halbs "half"), a word of no certain etymology. Perhaps from PIE root *skel- (1) "to cut," or perhaps a substratum word. Noun, adjective, and adverb all were in Old English.

Used also in Old English phrases, as in modern German, to mean "one half unit less than," for example þridda healf "two and a half," literally "half third." The construction in two and a half, etc., is first recorded c. 1200. Of time, in half past ten, etc., first attested 1750; in Scottish, the half often is prefixed to the following hour (as in German, halb elf = "ten thirty").

To go off half-cocked in the figurative sense "speak or act too hastily" (1833) is in allusion to firearms going off prematurely; half-cocked in a literal sense "with the cock lifted to the first catch, at which position the trigger does not act" is recorded by 1750. In 1770 it was noted as a synonym for "drunk." Bartlett ("Dictionary of Americanisms," 1848) writes that it was "a metaphorical expression borrowed from the language of sportsmen, and is applied to a person who attempts a thing in a hurry without due preparation, and consequently fails."

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.