half-truth (n.)

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."

Old English triewð (West Saxon), treowð (Mercian) "faith, faithfulness, fidelity, loyalty; veracity, quality of being true; pledge, covenant," from Germanic abstract noun *treuwitho, from Proto-Germanic treuwaz "having or characterized by good faith," from PIE *drew-o-, a suffixed form of the root *deru- "be firm, solid, steadfast." With Germanic abstract noun suffix *-itho (see -th (2)).

Sense of "something that is true" is first recorded mid-14c. Meaning "accuracy, correctness" is from 1560s. English and most other IE languages do not have a primary verb for "speak the truth," as a contrast to lie (v.). Truth squad in U.S. political sense first 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]

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