"apparent telepathic vibration transferring a thought from one person to another without any other medium," 1869, from brain (n.) + wave (n.).
Entries linking to brain-wave
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), 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."
wave n.
"moving billow of water," 1520s, alteration (by influence of wave (v.)) of Middle English waw, which is from Old English wagian "to move to and fro," from Proto-Germanic *wag- (source also of Old Saxon, Old High German wag, Old Frisian weg, Old Norse vagr "water in motion, wave, billow," Gothic wegs "tempest"), probably from PIE root *wegh- "to go, move." The usual Old English word for "moving billow of water" was yð.
The "hand motion" meaning is recorded from 1680s; meaning "undulating line" is recorded from 1660s. Of people in masses, first recorded 1852; in physics, from 1832. Sense in heat wave is from 1843. The crowd stunt in stadiums is attested under this name from 1984, the thing itself said to have been done first Oct. 15, 1981, at the Yankees-A's AL championship series game in the Oakland Coliseum; soon picked up and popularized at University of Washington. To make waves "cause trouble" is attested from 1962.