The Collaborative International Dictionary of English v.0.48:

Fool \Fool\, n. [OE. fol, n. & adj., F. fol, fou, foolish, mad;
   a fool, prob. fr. L. follis a bellows, wind bag, an inflated
   ball; perh. akin to E. bellows. Cf. Folly, Follicle.]
   1. One destitute of reason, or of the common powers of
      understanding; an idiot; a natural.
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   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.
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            Extol not riches, then, the toil of fools. --Milton.
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            Experience keeps a dear school, but fools will learn
            in no other.                          --Franklin.
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   3. (Script.) One who acts contrary to moral and religious
      wisdom; a wicked person.
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            The fool hath said in his heart, There is no God.
                                                  --Ps. xiv. 1.
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   4. One who counterfeits folly; a professional jester or
      buffoon; a retainer formerly kept to make sport, dressed
      fantastically in motley, with ridiculous accouterments.
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            Can they think me . . . their fool or jester?
                                                  --Milton.
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   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.
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WordNet (r) 3.0 (2006):

Aethusa cynapium
    n 1: European weed naturalized in America that resembles parsley
         but causes nausea and poisoning when eaten [syn: fool's
         parsley, lesser hemlock, Aethusa cynapium]