Ac″id (�), a. [L. acidus sour, fr. the root ak to be sharp: cf. F. acide. Cf. Acute.] 1. Sour, sharp, or biting to the taste; tart; having the taste of vinegar: as, acid fruits or liquors. Also fig.: Sour-tempered.
He was stern and his face as acid as ever.
A. Trollope.
2. Of or pertaining to an acid; as, acid reaction.