also birdbrain, 1936, slang, "stupid person," also perhaps suggestive of flightiness, from bird (n.1) + brain (n.). Bird-brained is attested from 1910 and bird-witted from c. 1600.

"group of experts assembled to give advice on some matter," occasionally used since early 1900s, it became current in 1933, in reference to the intellectuals gathered by the administration of incoming U.S. President Franklin D. Roosevelt as advisers; from brain (n.) + trust (n.).

"idea, creation of one's own mind," 1850, from brain (n.) + child. Earlier was the alliterative brain-brat (1630).

popular name of a type of coral, 1709, from brain (n.) + coral; so called for its appearance.

"suffering complete loss of brain functioning," 1971 (brain death is from 1968), from brain (n.) + dead. Popularized in U.S. 1975 by journalistic coverage of the Karen Anne Quinlan case.

"emigration of experts and trained people to richer countries from poorer ones," 1963, from brain (n.) + drain (n.).

"very smart person," 1982, U.S. slang, from brain (n.) + ending from ENIAC, etc. Brainiac also was the name of a comic-book villain in the Superman series and a do-it-yourself computer building kit, both from the late 1950s, and the word may bear traces of either or both of these.

late 15c., "witless, stupid," from brain (n.) + -less. Related: Brainlessly; brainlessness.

also brainrot, "decomposition in the citadel of life" (Algernon T.B. de Bale, M.D., "Exhausted Brain," 1879); coined 1854 by Thoreau, in "Walden," evidently based on potato-rot (blight):

While England endeavours to cure the potato-rot, will not any endeavour to cure the brain-rot, which prevails so much more widely and fatally ?

From a paragraph that opens "Why level downward to our dullest perception always, and praise that as common-sense? The commonest sense is the sense of men asleep, which they express by snoring." See brain (n.) + rot (n.).

"central trunk of a mammal's brain," 1875, from German; see brain (n.) + stem (n.).