Origin and history of trustworthy

c. 1200, "reliance on the veracity, integrity, or other virtues or sound principles of someone or something; religious faith," probably 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," Danish trøst, Gothic trausti "agreement, alliance").

This is reconstructed to be from Proto-Germanic *treuwaz, source of Old English treowian "to believe, trust," and treowe "faithful, trusty" (from PIE root *deru- "be firm, solid, steadfast;" compare trow (v.), true (adj.)).

It is attested from c. 1300 as "reliability, trustworthiness; trustiness, fidelity, faithfulness;" from late 14c. as "confident expectation" and "that on which one relies."

It is recorded from early 15c. in the legal sense of "confidence placed in a person who holds or enjoys the use of property entrusted to him by its legal owner;" and by mid-15c. as "condition of being legally entrusted," hence "that which is committed to one for safekeeping or use."

The meaning "businesses organized to reduce competition; an organization for the control of several corporations under one direction" by a set of majority shareholders in all of them, is a legal sense attested by 1877 which exploded as a U.S. political issue, hence trust-buster, recorded by 1903.

mid-13c., worthi, "important, good, having merit;" c. 1300, "deserving of reverence;" from worth (n.) + -y (2). Also sometimes in Middle English "having great monetary value."

"Of sufficient importance," hence extensive use from 13c. in compounds (seaworthy, love-worthy, etc.),

It is attested from late 14c. as a noun, "person of eminent worth," 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. Worthful (Old English weorþful) was "valued, precious;" also "glorious, esteemed," but it vanished in early Middle English.

"unreliable, not trustworthy" in any sense, 1846, from un- (1) "not" + trustworthy. Related: Untrustworthiness (1808). The earlier word in the same sense was untrusty (late 14c.). Middle English also had a verb untrust "not trust" (someone, oneself) and a noun untrust "distrust, lack of confidence" (c. 1200); "untrustworthiness" (mid-15c.).