Origin and history of brain-stem

"soft, grayish mass filling the cranial cavity of a vertebrate," in the broadest sense, "organ of consciousness and the mind," Old English brægen "brain," from Proto-Germanic *bragnan (source also of Middle Low German bregen, Old Frisian and Dutch brein), a word of uncertain origin, perhaps from PIE root *mregh-m(n)o- "skull, brain" (source also of Greek brekhmos "front part of the skull, top of the head").

But Liberman writes that brain "has no established cognates outside West Germanic" and is not connected to the Greek word. More probably, he writes, its etymon is PIE *bhragno "something broken."

The custom of using the plural to refer to the substance (literal or figurative), as opposed to the organ, dates from 16c. The figurative sense of "intellectual power" is from late 14c.; the meaning "a clever person" is recorded by 1914.

To have something on the brain "be extremely eager for or interested in" is from 1862. Brain-fart "sudden loss of memory or train of thought; sudden inability to think logically" is by 1991 (brain-squirt is from 1650s as "feeble or abortive attempt at reasoning"). An Old English word for "head" was brægnloca, which might be translated as "brain locker." In Middle English, brainsick (Old English brægenseoc) meant "mad, addled."

[main body of a tree] Middle English stemme, from Old English stemn, stefn "trunk of a tree or shrub," the part which rises from the ground and supports the branches; also "either end-post of a ship;" from Proto-Germanic *stamniz (source also of Old Saxon stamm, Old Norse stafn "stem of a ship;" Danish stamme, Swedish stam "trunk of a tree;" Old High German stam, German Stamm). This is thought to be from a suffixed form of the PIE root *sta- "to stand, make or be firm."

Especially of the post at the bow of a ship, hence the word came to mean "front of a ship" generally by 1550s. That sense is preserved in the phrase stem to stern, which is originally nautical, "along the full length" (of a ship), and is attested from 1620s.

By 1590s as "stalk which supports the flower of a plant;" the meaning "support of a wineglass" is by 1835. Extended to other things resembling the stem of a plant; in type-founding, "thick stroke of a letter" (1670s); stems as slang for "legs" is by 1860.

The sense of "stock of a family, ancestry" is attested by 1530s; the sense also is in Middle High German stam, Dutch stam; Old High German stam is only in the literal sense, but it meant "race" in compound liut-stam; for which also compare Old English leodstefn "race."

In modern linguistics, the sense of "part of a word that remains unchanged through inflection" is from 1830. In biology, stem cell is attested by 1885.