The Collaborative International Dictionary of English v.0.48:

Foolish \Fool"ish\, a.
   1. Marked with, or exhibiting, folly; void of understanding;
      weak in intellect; without judgment or discretion; silly;
      unwise.
      [1913 Webster]

            I am a very foolish fond old man.     --Shak.
      [1913 Webster]

   2. Such as a fool would do; proceeding from weakness of mind
      or silliness; exhibiting a want of judgment or discretion;
      as, a foolish act.
      [1913 Webster]

   3. Absurd; ridiculous; despicable; contemptible.
      [1913 Webster]

            A foolish figure he must make.        --Prior.

   Syn: Absurd; shallow; shallow-brained; brainless; simple;
        irrational; unwise; imprudent; indiscreet; incautious;
        silly; ridiculous; vain; trifling; contemptible. See
        Absurd.
        [1913 Webster]

WordNet (r) 3.0 (2006):

foolish
    adj 1: devoid of good sense or judgment; "foolish remarks"; "a
           foolish decision" [ant: wise]
    2: having or revealing stupidity; "ridiculous anserine
       behavior"; "a dopey answer"; "a dopey kid"; "some fool idea
       about rewriting authors' books" [syn: anserine, dopy,
       dopey, foolish, goosey, goosy, gooselike, jerky]

Moby Thesaurus II by Grady Ward, 1.0:

194 Moby Thesaurus words for "foolish":
   Pickwickian, absurd, airy, apish, arch, asinine, batty, befooled,
   beguiled, bemused, besotted, beyond belief, bizarre, blockheaded,
   boneheaded, brainless, brash, buffoonish, catchpenny, cockamamie,
   cockeyed, confused, crazy, credulous, cuckoo, daffy, daft, dazed,
   demented, devilish, dim, dim-witted, dippy, dizzy, dopey, doting,
   dotty, dull-witted, dumb, elfish, elvish, empty, empty-headed,
   erratic, extravagant, fantastic, fatuitous, fatuous, feebleminded,
   flaky, flimsy, fond, fool, foolhardy, foolheaded, fribble,
   fribbling, frivolous, frothy, fuddled, full of mischief, futile,
   gaga, goofy, grotesque, gulled, hasty, headless, headlong,
   heedless, high-flown, high-spirited, idiotic, idle, ignorant,
   ill-advised, ill-considered, illogical, imbecile, imbecilic,
   impetuous, impish, impolitic, imprudent, inane, incautious,
   incredible, indiscreet, inept, infatuated, injudicious, insane,
   insensate, irrational, knavish, kooky, lackbrained, laughable,
   lean-minded, lean-witted, light, light-headed, loony, ludicrous,
   mad, maudlin, mindless, mischief-loving, mischievous, misguided,
   monstrous, moronic, nonsensical, not bright, nugacious, nugatory,
   numskulled, nuts, nutty, obtuse, of little brain, otiose,
   outrageous, outre, pea-brained, pin-brained, playful, poppycockish,
   potty, prankish, pranksome, pranky, preposterous, puckish, rash,
   reasonless, reckless, ridiculous, roguish, rubbishy, sappy,
   scampish, scapegrace, scatterbrained, screwy, senseless,
   sentimental, shallow, short-sighted, silly, skimble-skamble,
   slackminded, slackwitted, slender, slight, slow-witted, sportive,
   stolid, stupid, superficial, thick, thickheaded, thoughtless,
   trashy, trickish, tricksy, trifling, trite, trivial, twaddling,
   twaddly, ungifted, unintellectual, unintelligent, unorthodox,
   unreasonable, unreasoning, unstable, untalented, unthinking,
   unwary, unwise, unwitty, vacuous, vain, vapid, wacky, waggish,
   weak-minded, weird, wet, wild, windy, witless