The Collaborative International Dictionary of English v.0.48:

Silly \Sil"ly\, a. [Compar. Sillier; superl. Silliest.] [OE.
   seely, sely, AS. s?lig, ges?lig, happy, good, fr. s?l, s?l,
   good, happy, s?l good fortune, happines; akin to OS.
   s[=a]lig, a, good, happy, D. zalig blessed, G. selig, OHG.
   s[=a]l[imac]g, Icel. s?l, Sw. s[aum]ll, Dan. salig, Goth.
   s?ls good, kind, and perh. also to L. sollus whole, entire,
   Gr. ???, Skr. sarva. Cf. Seel, n.]
   1. Happy; fortunate; blessed. [Obs.] --Chaucer.
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   2. Harmless; innocent; inoffensive. [Obs.] "This silly,
      innocent Custance." --Chaucer.
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            The silly virgin strove him to withstand. --Spenser.
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            A silly, innocent hare murdered of a dog. --Robynson
                                                  (More's
                                                  Utopia).
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   3. Weak; helpless; frail. [Obs.]
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            After long storms . . .
            With which my silly bark was tossed sore. --Spenser.
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            The silly buckets on the deck.        --Coleridge.
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   4. Rustic; plain; simple; humble. [Obs.]
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            A fourth man, in a sillyhabit.        --Shak.
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            All that did their silly thoughts so busy keep.
                                                  --Milton.
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   5. Weak in intellect; destitute of ordinary strength of mind;
      foolish; witless; simple; as, a silly woman.
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   6. Proceeding from want of understanding or common judgment;
      characterized by weakness or folly; unwise; absurd;
      stupid; as, silly conduct; a silly question.
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   Syn: Simple; brainless; witless; shallow; foolish; unwise;
        indiscreet. See Simple.
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WordNet (r) 3.0 (2006):

silly
    adj 1: ludicrous, foolish; "gave me a cockamamie reason for not
           going"; "wore a goofy hat"; "a silly idea"; "some wacky
           plan for selling more books" [syn: cockamamie,
           cockamamy, goofy, sappy, silly, wacky,
           whacky, zany]
    2: lacking seriousness; given to frivolity; "a dizzy blonde";
       "light-headed teenagers"; "silly giggles" [syn: airheaded,
       dizzy, empty-headed, featherbrained, giddy, light-
       headed, lightheaded, silly]
    3: inspiring scornful pity; "how silly an ardent and
       unsuccessful wooer can be especially if he is getting on in
       years"- Dashiell Hammett [syn: pathetic, ridiculous,
       silly]
    4: dazed from or as if from repeated blows; "knocked silly by
       the impact"; "slaphappy with exhaustion" [syn: punch-drunk,
       silly, slaphappy]
    n 1: a word used for misbehaving children; "don't be a silly"

Moby Thesaurus II by Grady Ward, 1.0:

154 Moby Thesaurus words for "silly":
   Pickwickian, absurd, airy, apish, asinine, balmy, batty, bedazzled,
   befooled, beguiled, benumbed, besotted, blockhead, bonehead, booby,
   brainless, buffoonish, catchpenny, childish, clod, clot, cockeyed,
   crazy, credulous, daffy, daft, daze, dazed, dazzled, dippy, dizzy,
   dolt, dope, dopey, doting, drip, dumb, dummy, dunce, empty,
   empty-headed, fatuitous, fatuous, featherbrained, flaky, flighty,
   flimsy, fond, fool, foolhardy, foolheaded, foolish, fribble,
   fribbling, frivolous, frothy, fuddled, futile, gaga, giddy, goofy,
   goose, groggy, gulled, harebrained, idiot, idiotic, idle,
   ignoramus, ignorant, illogical, imbecile, imbecilic, in a daze,
   inane, inept, infatuated, insane, irrational, irresponsible,
   jackass, jerk, knocked silly, kooky, laughable, light,
   light-headed, loony, ludicrous, mad, maudlin, mazed, moronic,
   muzzy, nerd, nincompoop, ninny, nitwit, nitwitted, nonsensical,
   nugacious, nugatory, numskull, nutty, off, otiose, pointless,
   poppycockish, preposterous, puerile, punch-drunk, punchy,
   rattlebrained, ridiculous, risible, rubbishy, sappy,
   scatterbrained, screwy, senseless, sentimental, shallow,
   silly Billy, silly ass, simpleton, skimble-skamble, slaphappy,
   slender, slight, stunned, stupefied, stupid, superficial,
   thoughtless, trashy, trifling, trite, trivial, twaddling, twaddly,
   twit, unintelligent, unreasonable, unwise, unwitty, vacuous, vain,
   vapid, wacky, weak-minded, wet, windy, witless, woozy