acid (adj.) Look up acid at
1620s, "of the taste of vinegar," from French acide (16c.) or directly from Latin acidus "sour, sharp," adjective of state from acere "to be sour," from PIE root *ak- "sharp, pointed" (see acrid). Figurative use from 1775; applied to intense colors from 1916. Acid test is American English, 1892, from the frontier days, when gold was distinguished from similar metals by application of nitric acid. Acid rain is first recorded 1859 in reference to England.
acid (n.) Look up acid at
1690s, from acid (adj.). Slang meaning "LSD-25" first recorded 1966 (see LSD).
When I was on acid I would see things that looked like beams of light, and I would hear things that sounded an awful lot like car horns. [Mitch Hedberg, 1968-2005, U.S. stand-up comic]
Acid rock (type played by or listen to by people using LSD) is also from 1966; acid house dance music style is 1988, probably from acid in the hallucinogenic sense + house "dance club DJ music style."