Green’s Dictionary of Slang

(drugs) the marijuana world’s equivalent of passive smoking; the sensations that a non-smoker can achieve through the simple act of being in the same room as those who smoke; thus inhaling the drug willy-nilly, as well as picking up on the particular atmosphere generated by a roomful of smokers; similarly ext. to LSD, MDMA etc.

1965		(con. 1940s) Malcolm X Autobiog. (1968) 187: He had smoked so many [reefers] through the bone that he could just light a match before the empty bone, draw the heat through, and get what he called a ‘contact’ high.
1970		Meyer & Ebert Beyond Valley of the Dolls [film script] Here, have some grass. [...] No, thanks, man. In a scene like this, you get a contact high!
1980		G. Liddy Will 112: With a groan the pooch rolled over, staggered to its feet, then lurched out of the room. It was on a contact high!
1994		Rebennack & Rummel Under A Hoodoo Moon 33: My mother would come in, and before long she’d be laughing and cutting up because she was getting contact high.
2004		Mad mag. Oct. 45: It’s the Amsterdam contact high.