Source : Webster's Revised Unabridged Dictionary (1913)

Silliness \Sil"li*ness\, n.
   The quality or state of being silly.

Source : WordNet®

silliness
     n 1: a ludicrous folly; "the crowd laughed at the absurdity of
          the clown's behavior" [syn: {absurdity}, {fatuity}, {fatuousness}]
     2: an impulsive scatterbrained manner [syn: {giddiness}]