The Collaborative International Dictionary of English v.0.48:

Tomfoolery \Tom`fool"er*y\, n.
   Trifling or silly behavior done mostly for amusement;
   foolishness; fooling around.

   Syn: tomfoolishness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

tomfoolery
    n 1: foolish or senseless behavior [syn: folly, foolery,
         tomfoolery, craziness, lunacy, indulgence]