Source : Webster's Revised Unabridged Dictionary (1913)

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[=i]g, Icel. s?l, Sw. s["a]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.

   2. Harmless; innocent; inoffensive. [Obs.] ``This silly,
      innocent Custance.'' --Chaucer.

            The silly virgin strove him to withstand. --Spenser.

            A silly, innocent hare murdered of a dog. --Robynson
                                                  (More's
                                                  Utopia).

   3. Weak; helpless; frail. [Obs.]

            After long storms . . . With which my silly bark was
            tossed sore.                          --Spenser.

            The silly buckets on the deck.        --Coleridge.

   4. Rustic; plain; simple; humble. [Obs.]

            A fourth man, in a sillyhabit.        --Shak.

            All that did their silly thoughts so busy keep.
                                                  --Milton.

   5. Weak in intellect; destitute of ordinary strength of mind;
      foolish; witless; simple; as, a silly woman.

   6. Proceeding from want of understanding or common judgment;
      characterized by weakness or folly; unwise; absurd;
      stupid; as, silly conduct; a silly question.

   Syn: Simple; brainless; witless; shallow; foolish; unwise;
        indiscreet. See {Simple}.

Source : WordNet®

silly
     adj 1: pungent adjectives of disesteem; "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}, {wacky},
            {whacky}, {zany}, {unreasonable}]
     2: lacking seriousness; given to frivolity; "a dizzy blonde";
        "light-headed teenagers"; "silly giggles" [syn: {airheaded},
         {dizzy}, {empty-headed}, {featherbrained}, {giddy}, {light-headed},
         {lightheaded}]
     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}]
     4: dazed from or as if from repeated blows; "knocked silly by
        the impact"; "slaphappy with exhaustion" [syn: {punch-drunk},
         {slaphappy}]
     n : a word used for misbehaving children; "don't be a silly"
     [also: {silliest}, {sillier}]