trustworthy adj.

c. 1200, "reliance on the veracity, integrity, or other virtues of someone or something; religious faith," from Old Norse traust "help, confidence, protection, support," from Proto-Germanic abstract noun *traustam (source also of Old Frisian trast, Dutch troost "comfort, consolation," Old High German trost "trust, fidelity," German Trost "comfort, consolation," Gothic trausti "agreement, alliance"), from Proto-Germanic *treuwaz, source of Old English treowian "to believe, trust," and treowe "faithful, trusty," from PIE root *deru- "be firm, solid, steadfast."

from c. 1300 as "reliability, trustworthiness; trustiness, fidelity, faithfulness;" from late 14c. as "confident expectation" and "that on which one relies." From early 15c. in legal sense of "confidence placed in a one who holds or enjoys the use of property entrusted to him by its legal owner;" mid-15c. as "condition of being legally entrusted." Meaning "businesses organized to reduce competition" is recorded from 1877. Trust-buster is recorded from 1903.

mid-13c., "having merit," from worth (n.) + -y (2). Old English had weorþful in this sense. Attested from late 14c. as a noun meaning "person of merit" (especially in Nine Worthies, famous men of history and legend: Joshua, David, Judas Maccabæus, Hector, Alexander, Julius Cæsar, Arthur, Charlemagne, Godfrey of Bouillon -- three Jews, three gentiles, three Christians). Related: Worthily; worthiness.

1846, from un- (1) "not" + trustworthy. Related: Untrustworthiness.