in truth

Old English triewð (West Saxon), treowð (Mercian) "faithfulness, quality of being true," from triewe, treowe "faithful" (see true), with Proto-Germanic abstract noun suffix *-itho (see -th (2)).

Meaning "accuracy, correctness" is from 1560s. Unlike lie (v.), there is no primary verb in English or most other IE languages for "speak the truth." Noun sense of "something that is true" is first recorded mid-14c.

Truth squad in U.S. political sense first attested 1952. Truthiness "act or quality of preferring concepts or facts one wishes to be true, rather than those known to be true," catch word popularized in this sense by U.S. comedian Stephen Colbert, declared by American Dialect Society to be "2005 Word of the Year."