Fool, n.
 1. One destitute of reason, or of the common powers of understanding; an idiot; a natural.
 2. A person deficient in intellect; one who acts absurdly, or pursues a course contrary to the dictates of wisdom; one without judgment; a simpleton; a dolt.
              Extol not riches, then, the toil of fools.   --Milton.
    Experience keeps a dear school, but fools will learn in no other.   --Franklin.
 3. Script. One who acts contrary to moral and religious wisdom; a wicked person.
       The fool hath said in his heart, There is no God.   --Ps. xiv. 1.
 4. One who counterfeits folly; a professional jester or buffoon; a retainer formerly kept to make sport, dressed fantastically in motley, with ridiculous accouterments.
            Can they think me . . . their fool or jester?   --Milton.
 April fool, Court fool, etc. See under April, Court, etc.
 Fool's cap, a cap or hood to which bells were usually attached, formerly worn by professional jesters.
 Fool's errand, an unreasonable, silly, profitless adventure or undertaking.
 Fool's gold, iron or copper pyrites, resembling gold in color.
 Fool's paradise, a name applied to a limbo (see under Limbo) popularly believed to be the region of vanity and nonsense. Hence, any foolish pleasure or condition of vain self-satistaction.
 Fool's parsley Bot., an annual umbelliferous plant (Aethusa Cynapium) resembling parsley, but nauseous and poisonous.
 To make a fool of, to render ridiculous; to outwit; to shame. [Colloq.]
 To play the fool, to act foolishly; to act the buffoon; to act a foolish part. “I have played the fool, and have erred exceedingly.” --1 Sam. xxvi. 21.


 fool's errand
      n : a fruitless mission