What word or phrase does your mother always say?
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.
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 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."
Used in various senses in Scripture. In Prov. 12:17, 19, it denotes that which is opposed to falsehood. In Isa. 59:14, 15, Jer. 7:28, it means fidelity or truthfulness. The doctrine of Christ is called "the truth of the gospel" (Gal. 2:5), "the truth" (2 Tim. 3:7; 4:4). Our Lord says of himself, "I am the way, and the truth" (John 14:6).