acid [E17th] Acid originally meant ‘sour-tasting’ and came from Latin acidus. The term seems to have been introduced by the scientist Francis Bacon, who in 1626 described sorrel as ‘a cold and acid herb’. The chemical sense developed in the middle of that century because most common acids taste sour. The acid test [M18th] was originally a method of testing gold using nitric acid. An object made of gold will show no sign of corrosion if immersed in nitric acid, unlike one made of another metal. By the mid 19th century the expression had come to mean any situation that proves a person’s or thing’s quality. Acerbic [M19th] is from Latin acerbus ‘sour-tasting’, and acrid [M17th] is from the related Latin acer ‘sharp, pungent’ with spelling influenced by acid.