Green’s Dictionary of Slang

(US) a very clever person.

1984		L.A. Times 18 June 155/1: I have friends with you-must lists: you must be some braino, you must never go out, you must not have a boyfriend.
1984		W. Safire Take My Word For It 242: A term of opprobrium for intellectuals is braino.
1986		R. Walker AF Wives 164: They were grinds — what they called ‘brainos’ at the academy [HDAS].
2001		North Harris College 🌐 North Harris College math students! E-mail Dr. Braino with your homework questions! If it’s a really good question or problem, ‘Dr. Braino’ will post the question and answer here for all to read!