Green’s Dictionary of Slang

(drugs) the repetition after the event, typically without the presence of any drug, of the emotions, sensations or hallucinations of a previous LSD experience; also in non-drug use.

1970		Amer. Annual 486: This decline has been due, in part, to the bad ‘trips’ that terrified many users, the requency of disturbing ‘flashbacks,’ and the prolonged anxiety states and psychotic reactions that were experienced.
1981		D.E. Miller Bk of Jargon 341: flashback: The recurrence of effects from a previously taken drug, especially LSD or other hallucinogens. Flashbacks can sometimes occur months after a person’s drug use has been terminated.
1984		(con. 1964–73) W. Terry Bloods (1985) 33: [of military service] What y’all talking about [...] You fired guns from five miles away and talking ’bout flashbacks?
1992		in R. Graef Living Dangerously 95: I’ve never had a bad trip and [...] I’ve never had a flashback either.
2002		Fabian & Byrne Out of Time (ms.) 112: Instead of having a flash-back, I was having a flash-forward.