Merriam-Webster Collegiate Dictionary - acid

 I. adjective  Etymology: French or Latin; French ~e, from Latin ~us, from acere to be sour — more at acet-  Date: 1626  1.  a. sour, sharp, or biting to the taste  b. sharp, biting, or sour in manner, disposition, or nature an ~ individual  c. sharply clear, discerning, or pointed an ~ wit  d. piercingly intense and often jarring ~ yellow  2.  a. of, relating to, or being an ~; also having the reactions or characteristics of an ~ ~ soil an ~ solution  b. of salts and esters derived by partial exchange of replaceable hydrogen ~ sodium carbonate NaHCO3  c. containing or involving the use of an ~ (as in manufacture)  d. marked by or resulting from an abnormally high concentration of ~ ~ indigestion  3. relating to or made by a process (as in making steel) in which the furnace is lined with ~ic material and an ~ic slag is used  4. rich in silica ~ rocks  • ~ly adverb  • ~ness noun  II. noun  Date: 1696  1. a sour substance; specifically any of various typically water-soluble and sour compounds that in solution are capable of reacting with a base to form a salt, redden litmus, and have a pH less than 7, that are hydrogen-containing molecules or ions able to give up a proton to a base, or that are substances able to accept an unshared pair of electrons from a base  2. something incisive, biting, or sarcastic a social satire dripping with ~  3. LSD  • ~y adjective