資料來源 : Webster's Revised Unabridged Dictionary (1913)

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.

   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
      ({[AE]thusa 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 the buffoon; to act a foolish
      part. ``I have played the fool, and have erred
      exceedingly.'' --1 Sam. xxvi. 21.

資料來源 : WordNet®

fool's errand
     n : a fruitless mission