1620s, "of the taste of vinegar," from French acide (16c.) or directly from Latin acidus "sour, sharp, tart" (also figurative, "disagreeable," etc.), adjective of state from acere "to be sour, be sharp," from PIE root *ak- "be sharp, rise (out) to a point, pierce."
Figurative use in English is from 1775; applied to intense colors from 1916; an acid dye (1888) involves an acid bath. Acid test is American English, 1881, a quick way to distinguish gold from similar metals by application of nitric acid. Acid rain "highly acidity in rain caused by atmospheric pollution" is first recorded 1859 in reference to England. Acid drop as a kind of hard sugar candy flavored with tartaric acid is by 1835, with drop (n.) in the "lozenge" sense.