Green’s Dictionary of Slang

a really clever person.

1983		R. Price Breaks 107: The kid was a brainiac. Bronx Science, NYU.
1998		‘Bug World Awards’ in Shepherd Express (Milwaukee) 30 Jul. XIX:31 🌐 She’s Sally, working at a casino oyster bar, yearning to become the first female blackjack dealer in Monte Carlo. She’s not exactly a brainiac, but getting by as best she can.
1999		Guardian Guide 5–12 June 87: The odious Nash [...] meets his match in a loony-tunes brainiac.
2002		Mad mag. July 27: Maybe if you damn brainiacs accepted any of my words.