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," 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]