Acid  —n. 1 a any of a class of substances that liberate hydrogen ions in water, are usu. Sour and corrosive, turn litmus red, and have a ph of less than 7. B any compound or atom donating protons. 2 any sour substance. 3 slang the drug lsd. —adj. 1 sour. 2 biting, sharp (an acid wit). 3 chem. Having the essential properties of an acid.  acidic adj. Acidify v. (-ies, -ied). Acidity n. Acidly adv. [latin aceo be sour]