Reliance that later named a legal hold
In everyday English, trust is the confidence you place in a person, a process, or a promise: that someone will tell the truth, keep a secret, or not fail under pressure. The same noun also names a legal arrangement in which one party holds property for another, and, in older commercial talk, a combination of firms that tries to choke competition. Those later senses grew out of the first. They did not replace it.
The origin of the word trust is harder to pin down than the modern meanings suggest. English writers have used the noun and the verb since about 1200, in the Middle English period, with senses already close to reliance and having faith. What is still argued is whether the form itself was borrowed from Scandinavia or inherited from a lost Old English word.
The Norse look-alike and the vowel problem
For a long time, dictionaries treated Middle English trust (also spelled trost, trist, and similar) as a loan from Old Norse traust, a noun meaning help, confidence, protection, or support. The match in sense is close, and the Norse verb treysta, to rely on or to make strong and safe, looks like a ready source for English trust as a verb. American Heritage still hedges in that direction. The Middle English Dictionary does too.
The trouble is the vowel. Old Norse au does not normally yield the short u of English trust. Scholars who work the Scandinavian element in English, including the Gersum Project and the current Oxford English Dictionary, therefore prefer another account: an unattested Old English noun *trust, from a rare zero-grade variant of the same Germanic root, *trust- beside the better-attested *traust-. That reconstruction would give the English vowel for free. It also finds possible relatives outside Scandinavia: Middle High German getrüste, a host or troop, and the Frankish source of medieval Latin trustis, a retinue or bodyguard in the Salic law. Those forms point to a native West Germanic presence, not only a Norse import.
The two stories are not as far apart as they sound. Everyone agrees that English trust belongs with a Germanic family of words for firmness, help, and fidelity. The dispute is only whether the English form came across the North Sea in the Viking period or was already sitting in Old English under a spelling we happen not to have.
Firmness at the root
There is no living prefix or suffix to peel off in modern trust. The morphology sits further back. Proto-Germanic is reconstructed with an abstract noun *traustą and with the adjective *treuwaz, characterized by good faith. From the latter come Old English treowe, faithful or trustworthy, and treowian, to believe or trust, which survive in English as true, truth, and the now literary trow.
Those Germanic forms are traced to the Proto-Indo-European root *deru- (also given as *dreu-), to be firm, solid, and steadfast. The same root specialized in another direction as wood and tree, which is why English tree sits in the same deep family as true and trust. That is a reconstructed kinship, not a claim that trust once meant timber.
Cognates make the range of meaning visible. Icelandic still has traust for faith or confidence. Dutch troost and German Trost mean comfort or consolation: the same root, shifted toward the help you give someone who is shaken. Gothic trausti meant an agreement or alliance. Danish trøst and Swedish tröst keep the comfort sense. English kept the older social meaning: reliance on a person who will hold.
From faith to fiduciary duty
Around 1200 the noun already covered reliance on someone’s integrity and, in religious writing, faith. By about 1300 it could name reliability or faithfulness as a quality in the person trusted. Late in the fourteenth century it also meant confident expectation, and the thing on which one relies.
The legal sense is a later English development, not a separate borrowing. Early in the fifteenth century, trust described the confidence placed in a person who holds property for its legal owner. By the middle of that century it could mean the condition of being so entrusted, and then the property or duty itself. That path is almost visible in the ordinary verb: to trust someone with goods is already close to handing them a charge. English law named the arrangement after the confidence, then named the office and the estate after the arrangement.
Commerce followed the law. To buy on trust is to take credit: the seller relies on later payment. In the United States by 1877 the noun was already being used for a combination of businesses organized to reduce competition, with majority shareholders steering several corporations as one. That specialized legal sense became a political target; trust-buster is recorded by 1903. Computing later borrowed the word again for access rights, another case of who is allowed to hold what.
None of these later uses required a new etymology. They are the social meaning of reliance, applied to property, credit, cartels, and machines.
A cognate that drifted toward comfort
If you want a single reminder of how the family can move, set English trust beside German Trost. Both come from the Germanic stock that meant firmness and support. English kept the interpersonal wager: I rely on you. German, Dutch, and the Scandinavian consolation words kept the other half of the old sense, the help offered when someone needs to be made steady. The legal trust on an English deed and the Trost in a German hymn are not the same institution, but they are the same root seen from two sides: a hold that is supposed to be firm.