Green’s Dictionary of Slang

1. (US) an intellectual; also as a teasing term of address.

1937		(con. WW1) E.C. Parsons Great Adventure 288: [S]ome lame brain in the security of a comfortable office [...] thought up a brilliant new idea to give pursuit pilots a headache [...] this Einstein of the nonflying personnel at headquarters smacked his lips over the idea.
1967–8		Baker et al. CUSS.
2017		A. De Bruyn Escaping the Amazon 174: ‘How about we just buy a jalopy and drive to Venezuela’ [...] ‘Hey, Einstein, how will we cross that border without my passport?’.

2. (US campus) pubic hair.

1989		P. Munro Sl. U.
1997		K. Kainulainen ‘University Euphemisms in Calif. Today’ 🌐 A very noble-sounding ‘donor’ is a person who makes himself available for sexual intercourse, and scientific ‘Einstein’ means ‘pubic hair’.

3. in pl., brains.

1996

J. Cameron It Was An Accident 53: He got last in the queue when they gave out the Einsteins. Preferred doing everything you told him instead of starting on that thinking.