Green’s Dictionary of Slang

(US) to experience the effects of taking a drug, esp. hallucinatory.

1968		L. Wolf Voices from the Love Generation 17: They’ve been mind-fucked. Everyone who comes here is flashed, and gets flashed, and flashes.
1970		New Yorker 5 Dec. 215: If they are not stoned out of their skulls or have not flipped or flashed or freaked out, they rap about the people over thirty who have ruined their lives.
1971		E.E. Landy Underground Dict. (1972).
2019		D. Winslow Border [ebook] Ashamed, jonesing like hell, flashing like hell. The noise, the motion, the yelling..