maleic acid: a colorless crystalline compound found in unripe fruit (such as apples or tomatoes or cherries) and used mainly to make polyester resins
acid: 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
picket: a form of military punishment used by the British in the late 17th century in which a soldier was forced to stand on one foot on a pointed stake