Definitions of acid:

• noun:   street name for lysergic acid diethylamide
• noun:   any of various water-soluble compounds having a sour taste and capable of turning litmus red and reacting with a base to form a salt
• adjective:   containing acid

  Example: "An acid taste"

• adjective:   harsh or corrosive in tone

  Example: "A barrage of acid comments"