adjective

1. sharp and biting to the taste; sour; tart
2. sharp or sarcastic in temperament or speech
3. that is, or has the properties of, an acid
4. having too heavy a concentration of acid

Etymology: L acidus, sour < IE base *a-, sharp, pointed > ear

noun

1. a sour substance
2. Slang LSD
3. Chem. any compound that can react with a base to form a salt, the hydrogen of the acid being replaced by a positive ion; in modern theory, any substance that produces a positive ion and accepts electrons from a base to form covalent bonds: in water solution an acid tastes sour, turns blue litmus paper red, and, in the dissociation theory, produces free hydrogen ions