1640s, "holding something in trust," from Latin fiduciarius "entrusted, held in trust," from fiducia "trust, confidence, reliance;" in law, "a deposit, pledge, security," from root of fidere "to trust" (from PIE root *bheidh- "to trust, confide, persuade"). In Roman law, fiducia was "a right transferred in trust;" paper currency sense (1878) is because its value depends on the trust of the public. As a noun, "one who holds something in trust," from 1630s.
Proto-Indo-European root meaning "to trust, confide, persuade."
It forms all or part of: abide; abode; affiance; affidavit; auto-da-fe; bide; bona fide; confederate; confidant; confide; confidence; confident; defiance; defy; diffidence; diffident; faith; fealty; federal; federate; federation; fiancee; fideism; fidelity; fiducial; fiduciary; infidel; infidelity; nullifidian; perfidy; solifidian.
It is the hypothetical source of/evidence for its existence is provided by: Greek pistis "faith, confidence, honesty;" Latin fides "trust, faith, confidence, reliance, credence, belief;" Albanian be "oath," bindem "to be convinced, believe;" Old Church Slavonic beda "distress, necessity," bediti "to force, persuade;" Old English biddan "to ask, beg, pray," German bitten "to ask."