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.
      [1913 Webster]

   2. Harmless; innocent; inoffensive. [Obs.] "This silly,
      innocent Custance." --Chaucer.
      [1913 Webster]

            The silly virgin strove him to withstand. --Spenser.
      [1913 Webster]

            A silly, innocent hare murdered of a dog. --Robynson
                                                  (More's
                                                  Utopia).
      [1913 Webster]

   3. Weak; helpless; frail. [Obs.]
      [1913 Webster]

            After long storms . . .
            With which my silly bark was tossed sore. --Spenser.
      [1913 Webster]

            The silly buckets on the deck.        --Coleridge.
      [1913 Webster]

   4. Rustic; plain; simple; humble. [Obs.]
      [1913 Webster]

            A fourth man, in a sillyhabit.        --Shak.
      [1913 Webster]

            All that did their silly thoughts so busy keep.
                                                  --Milton.
      [1913 Webster]

   5. Weak in intellect; destitute of ordinary strength of mind;
      foolish; witless; simple; as, a silly woman.
      [1913 Webster]

   6. Proceeding from want of understanding or common judgment;
      characterized by weakness or folly; unwise; absurd;
      stupid; as, silly conduct; a silly question.
      [1913 Webster]

   Syn: Simple; brainless; witless; shallow; foolish; unwise;
        indiscreet. See Simple.
        [1913 Webster]