Is it ensure, insure, or assure?
c.1200, from Old Norse traust "help, confidence," from Proto-Germanic *traust- (cf. Old Frisian trast, Dutch troost "comfort, consolation," Old High German trost "trust, fidelity," German Trost "comfort, consolation," Gothic trausti "agreement, alliance"). Related to Old English treowian "to believe, trust," and treowe "faithful, trusty" (see true). Meaning "businesses organized to reduce competition" is recorded from 1877. Trust-buster is recorded from 1903.
early 13c., from Old Norse treysta "to trust," from traust (see trust (n.)). Related: Trusted; trusting.