English uses open for a door that is not shut, ground without cover, a speaker who hides nothing, a shop that is taking customers, and a contest that does not bar a class of players. The verb covers the physical act of unlatching and, later, the act of starting something. Those jobs look scattered, but they sit on one native Germanic adjective. English did not import this word from French or Latin; it inherited it.
Already English before the Conquest
The origin of the word open is a straight line from Old English. The adjective open is recorded in the Old English period, before 1150, with meanings that still feel close: a gate or eyelid not closed down, a place or body left exposed, a fact that is public or well known. That last sense could darken. Something open might be notorious rather than merely visible.
The matching verb is almost as old. Old English openian meant to cause a barrier to give way, to disclose or reveal, and, without an object, to become manifest or lie exposed. A prefixed twin, geopenian, covers much the same ground in the surviving texts: doors, mouths, boxes, wounds, and secrets. Historical linguists treat the adjective as older than the verb, even though both belong to the same early layer of the language.
Nothing later replaced them. Middle English kept the adjective, with spellings such as opyn and ope, and the verb openen. Romance rivals never pushed the native pair out of ordinary speech.
A raised lid, not an unlocked bolt
Behind Old English open stand Proto-West Germanic *opan and Proto-Germanic *upanaz. The usual reconstruction ties that form to the same prehistoric stock as English up: a Proto-Indo-European root *upo with the awkward range under, up from under, and therefore over. In Germanic the adjective looks like a past participle, as if a verb meaning to raise or set up had left a form for something already lifted. No such verb has been found. The shape is suggestive; the missing verb is a genuine gap.
If the reconstruction is right, the first picture is uncovering by lifting: a gate drawn up, a cover taken off, a surface left clear. Notes on *upanaz often sketch a path from lying on top to being clear or unobstructed. That is not the same mental image as a lock sliding back. Gothic, another early Germanic language, took a different route for the idea and used a word built as the reverse of shut. English and most of its West and North Germanic cousins kept the up-path instead.
The family is easy to see once the High German consonant shift is allowed for. Old High German offan became modern German offen, with p turned to ff. Dutch kept open. Old Norse opinn is still visible in Icelandic opinn, Swedish öppen, Danish åben, and Norwegian åpen. West Frisian has iepen; Scots has apen.
The verb is built on that adjective, not the other way around. Proto-Germanic *upanōną, Old English openian, German öffnen, Dutch openen, and the Scandinavian verbs of the öppna / åpne type are later formations. The suffix does ordinary work: it turns a state into an action. English open as a verb is that action, not a separate root.
What changed, and what did not
The physical core held. By about 1200 the adjective could describe unobstructed ground, not only a movable barrier. Rooms whose entrances were unclosed appear around 1300. Wounds that have not closed are late fourteenth century. Early in that century, frankness of speech is already a transferred sense: a person or a statement can be open because nothing is held back. Old English had already used the word for what is public or evident; Middle English simply spread that public quality onto character.
Later English kept adding objects without changing the root. Shops that are open, meaning available for trade, are nineteenth-century (from the 1820s). The verb gained a start-up sense in the 1690s: to open a proceeding is to set it in motion. In the twentieth century, to open up could mean to stop being secretive, from 1921. The noun followed its own track. Early thirteenth-century English used it for an aperture. The open as open country dates from the 1620s; open air is later still. Public knowledge, especially in out in the open, is modern (1942), though Middle English already had in open for something done in plain view.
Sports used the adjective socially rather than spatially. In 1861 Prestwick Golf Club declared that its championship belt would, from then on, be available to anyone in the world, after a first year limited to professionals. The noun an open, meaning a tournament not closed by class or invitation, is recorded from 1926 and first in golf. Tennis later named an era in which amateurs and professionals could meet in the same events. The word had not grown a new root; it had applied not-shut to who may enter.
Compounds keep the same logic. An open door in trade policy, from 1856, is a market not barred. Open season on game is nineteenth-century hunting language; the figurative use against people is early twentieth. An open book, of a person easy to read, is mid-nineteenth. An open house that welcomes any visitor is from the 1820s. None of these are fresh etymologies. They are the old adjective used on new things.
The cousin that still shows the lift
English speakers who want a reminder that open and up are kin can look at German. Offen is open; auf is up. Dutch does the same with open and op. Inside modern English the pair is easy to miss, because one word names a state and the other a direction. Historically they share the idea of something brought up from under cover: a lid raised, a fact no longer under wraps, a field with nothing in the way.
That is the residue worth keeping. The everyday adjective still looks like the past of a verb English never kept, and it still names the state of a door, a secret, and a championship that will take anyone who can play.