means the quality of consistently telling the truth and not deceiving others.

from From "truth" plus "-fulness." "Truth" descends from Old English "trēowþ" (also "trīewþ"), meaning faithfulness, fidelity, a solemn pledge — built on "trēowe," true, trusty, faithful, which is a cousin of words like "trust" and "troth" (as in "betrothed," pledging one's word). So buried in the modern noun is an older sense: not just accuracy of statement but the loyalty of someone whose word holds. The suffix "-ness" (Old English "-nes") simply turns the adjective into a quality — the state of being full of truth.