Green’s Dictionary of Slang

1. (drugs) a guide or caretaker during a hallucinogenic experience. Such a person is either not taking the drug or is veteran user.

1969		R.R. Lingeman Drugs from A to Z (1970).
1982		in AS LVII:4 289: A guide does not use drugs during parties but serves as a controller, known variously as groundman, babysitter, tour guide, co-pilot, ground control, guru, travel agent, or just sitter.

2. (US campus) an expert.

1986

Eble Campus Sl. Nov. 4: guru – an expert: My man is a bowling guru.