In current English, brain is first of all the organ inside the skull that integrates sensation, movement, and thought. The same noun also means mind or intelligence, and it can name a person who is unusually clever. Machines that steer or compute get the word by analogy. Those later senses matter, but they grew on a much older stem: English did not borrow this name from French or Latin. It inherited it.
From Old English into the present
The origin of the word brain is a story of inheritance. Old English already had brægen, also spelled brægn and bragen, a neuter noun for the cerebrum and cerebellum. It appears in medical writing and in glossaries that pair it with Latin cerebrum. One remedy in the Leechbook deals with a broken head: if the brain comes out, take the yolk of an egg. Another entry speaks of disease of the brain. The word was therefore the ordinary term for the organ well before the twelfth century, which is the dating modern dictionaries give for first known use only because that is how they mark pre-Conquest vocabulary.
Middle English carried the noun forward with the expected scatter of spellings: brain, brayn, braȝen, brein. Writers from Layamon to Chaucer use it. Nothing in that sequence looks like a loan. When later English needed a learned adjective, it turned to Latin cerebrum and produced cerebral. Greek enkephalos supplied encephalitis and related terms. Those bookish layers sit on top of the inherited noun; they never replaced it.
A verb followed the noun in the late fourteenth century: to brain someone was to smash the skull. The violent sense is still available, though the milder sense of hitting on the head is now more common in casual speech.
A North Sea family
The Old English forms go back to West Germanic bragna-, reconstructed further as Proto-Germanic bragną or bragnan. The close relatives are still transparent. Dutch has brein. West Frisian and older Frisian have brein. Low German has Brägen and Bregen. Scots has braine and brane. These are not chance lookalikes; they are the same West Germanic word in neighboring languages.
Standard German tells a different story. The usual words for the human organ are Hirn and Gehirn, from another Germanic root, the same stock that gave Old Saxon hirni. The old bragna- word did not become the default High German name for what sits in the human skull. It survives mainly as northern German Bregen or Brägen, used for the brain of a slaughtered animal, and in food names such as Bregenwurst. English kept the inherited noun for the organ of thought; much of High German reserved the cognate for the butcher's stall.
That split is old. Old Saxon, the continental relative of Old English, already preferred hirni. English, Frisian, Dutch, and Low German kept bragna-. The map of the word is therefore a North Sea map, not a pan-Germanic one.
Where the trail stops
Past West Germanic the etymology is disputed, and it should be left that way. A long-standing proposal links the Germanic noun to a Proto-Indo-European reconstruction along the lines of mregh-m(n)o-, glossed as skull or brain, and compares Greek brekhmos or bregma, the front of the skull or the crown of the head. Merriam-Webster still offers that Greek noun as a possible relative. Some reference works treat the match as a cognate.
Other specialists reject it. Anatoly Liberman has argued that brain has no established kin outside West Germanic, that the Greek comparison does not hold, and that a likelier older form would mean something broken. That alternative has not become a consensus reconstruction. Earlier attempts to tie the English word to Greek phrēn, a noun for midriff and mind, have been dropped.
What can be said without stretching is modest. The West Germanic family is secure. The Indo-European step is not. Modern English also offers no hidden morphology to unpack: brain is a simple inherited stem, not a prefix plus a root that speakers can still feel. Old compounds show how the noun behaved, not how it was built. Brægnloca named the head as a locker or enclosure for the brain. Brægenseoc, later brainsick, meant mad or addled. Both are transparent once the base noun is known.
Organ, substance, intellect
The physical meaning is the oldest and never left. The mental meanings grew beside it rather than replacing it. By the late fourteenth century the noun could already mean intellectual power. From the sixteenth century, English often used the plural brains for the substance, literal or figurative, as opposed to the organ counted as one. The jump from intelligence to a clever person is much later, recorded in the early twentieth century. Group uses such as the brains of an operation, the person who plans, follow the same path from tissue to talent.
Idioms keep the older picture in view. To have something on the brain, known from the nineteenth century, treats the organ as a place where an idea lodges. None of these extensions required a new etymology. They are the native noun doing more work.
A cognate on the butcher's board
The concrete remainder is the German cognate. English brain is an ordinary, even prestigious, word for the seat of thought. Its closest High German relative, Bregen, is at home in the vocabulary of slaughter and regional cookery. That contrast is not a joke about national character. It is what happens when two branches of West Germanic keep or discard the same inherited name. For a word so central to how English talks about thinking, the comparative evidence still ends at the North Sea, and the Greek link remains only a possibility.