Etymology
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brain (n.)

"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), 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. Figurative sense of "intellectual power" is from late 14c.; meaning "a clever person" is first recorded 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."

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brain (v.)
"to dash the brains out," late 14c., from brain (n.). Related: Brained; braining.
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brain-coral (n.)
1709, from brain (n.) + coral; so called for its appearance.
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lame-brain (n.)
"stupid person," 1921, from lame (adj.) + brain (n.).
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brain-stem (n.)
1875, from German; see brain (n.) + stem (n.).
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brain-teaser (n.)
"difficult puzzle or problem," 1893, from brain (n.) + agent noun from tease (v.).
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brain-drain (n.)
"emigration of experts and trained people to richer countries from poorer ones," 1963, from brain (n.) + drain (n.).
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brain-wave (n.)
"apparent telepathic vibration transferring a thought from one person to another without any other medium, 1869, from brain (n.) + wave (n.).
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brain-child (n.)
"idea, creation of one's own mind," 1850, from brain (n.) + child. Earlier was the alliterative brain-brat (1630).
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