LSD

[slang] acid [noun] [U] - an illegal drug which causes people who use it to see the world differently from the way it really is or to see things that do not really existIn the seventies, people were always telling stories about someone who jumped out of the window under the influence of LSD, thinking they could fly.They're on an LSD trip/They're tripping on LSD (= They're experiencing the effects of LSD).