A modal that does two jobs
In current English, must is a small, defective modal. It tells you that something is required: a law, a physical need, a social pressure, or a speaker’s insistence. It also does a second job that looks similar on the page and is not obligation at all: it marks an inference. If the lights are on, they must be home. The verb takes no -s in the third person, has no infinitive or participles of its own, and hands the past and the to-form to have to. Beside it stands a later noun: a map on a long hike, a vaccine before travel, a recording everyone talks about can each be a must — an item you cannot decently skip.
That cluster is native. English did not import the modal from French or Latin. It inherited a Germanic verb, then kept what had been the past tense and taught it to work as a present.
How a past form took over
The Old English verb was mōtan. In the present, a speaker said mōt: I may, I am allowed, I have the chance, or I am obliged. The past was mōste. The pattern belongs to the small class called preterite-present verbs. Their present forms had once been perfects, so they conjugated like the past tenses of strong verbs, then built a new weak past with a dental suffix. Can, may, and shall sit in the same group. Ought, like must, is historically a past that later served as a present.
From about 1300, English speakers used that old past as a present. The move is not mysterious. A past subjunctive often softens a current wish or request; over time the milder form can become the ordinary one. Middle English moste crowded out moten. The older present lingered as mote, now little more than a fossil in set phrases. Everyday English therefore kept only the past stem. That is why must looks the same in I must leave and in I knew that he must leave, and why a true past is usually had to.
Nothing in this path is a borrowing. The word is inherited from Proto-Germanic *mōtaną. Close relatives are easy to name: Old Saxon motan, Old Frisian mota, Dutch moeten, German müssen, Swedish måste (the Swedish form came in through Low German). Gothic preserves a prefixed cousin, gamōtan, with a sense that still surprises an English ear: to have room, to find space, to be able because there is room.
Room first, then permission, then duty
That Gothic meaning is the best window onto the older Germanic picture. If you have room — space, time, occasion — you may act. Old High German muozan and Old English mōtan both range across ability, permission, and obligation. German still keeps a noun from the same stock: Muße, leisure, originally a state that gives you the chance to do something. Middle Dutch had a similar noun. Necessity is not the starting point. Opportunity is. Duty is what happens when the chance hardens into a requirement you cannot decline.
How far back the root goes is unsettled, and the sources do not agree. Some handbooks derive the Germanic verb from an Indo-European root *med- meaning to measure or to take appropriate measures, as if the first idea were having an allotted amount of space or time. Other etymologists doubt any real link with the family of measure (German messen). A different proposal ties the verb to the Germanic noun for a meeting. The comparative Germanic story is solid. The Indo-European parent is not, and it is better to leave that gap open than to force a tidy tree.
There is no prefix or suffix to peel off in the modern English word. The history is inflectional. A preterite-present verb supplied a weak past mōste; that past became the only living form; English then used the same string as a noun and, later, in compounds.
From may to must, then to a thing you cannot miss
Inside English the sense-shift is easier to follow. Permission and compulsion sit close together: what you are allowed to do and what you cannot avoid doing have shared a verb in more than one Germanic language. Once must was locked in as a present modal of necessity, it also developed the epistemic reading — not you are required to, but this is the conclusion that fits the evidence. The two uses still live side by side. That is why must not is a prohibition, not a simple denial of need. Need not and do not have to do that other job.
The verb also became a noun by conversion. The Oxford English Dictionary records that use by the late sixteenth century. By the 1890s English has the familiar sense of something that has to be done, seen, or experienced; by the 1910s an adjective meaning indispensable is in play; mid-century compounds such as must-read follow. None of this restores an old infinitive. It simply turns a modal into a label for an essential item. An older literary habit, now rare, could even drop the following verb of motion: a speaker might say they must away, with go understood.
Lookalikes that are not the modal
English spelling hides several unrelated words. Unfermented grape juice, also called must, is already in Old English, taken from Latin mustum, short for vinum mustum, fresh wine, from mustus, new or fresh, perhaps originally wet. A stale smell called must is probably a back-formation from musty around 1600. Elephant musth, sometimes spelled must, is a nineteenth-century loan from Urdu mast, intoxicated, itself from Persian — another wet or drunken root, not the Germanic modal. Those lookalikes explain nothing about obligation. They only show how a short native past form and a Latin wine word happened to share a spelling.
What the cognates still remember
If one fact still cuts against the modern English verb, it is the Gothic one. Gamōtan is about having room. German Muße is leftover free time. Dutch moeten and German müssen, like English must, now mostly mean you have no choice. The family moved from space to permission to obligation. English went one step further: it threw away the present mōt, kept the past mōste, and taught that past to mean now. The origin of the word must is not a borrowed Latin duty-word. It is a native past tense that forgot it was past, after an older verb that once meant you had room to act.