1. LSD-25, the synthetic hallucinogenic drug. From the full name, Lysergic Acid Diethylamide. This has been the standard term by which users refer to the drug since its first popularity in California in 1965, in spite of the appearance of more picturesque but ephemeral alternatives.

In the late 1980s, adherents of the acid house cult adopted the word as a slogan (usually a cry of 'a-c-e-e-e-d!') and to refer to LSD or ecstasy.

2. British sarcasm, snide comments or cheeky exaggeration, especially in the expression 'come the old acid', popular in working class usage in the 1950s and 1960s and still heard. In such phrases as 'his acid comments' the adjectival mean- ing is similar, but cannot be described as slang.

Don't come the old acid with me!