The Collaborative International Dictionary of English v.0.48:

Merriness \Mer"ri*ness\, n.
   The quality or state of being merry; merriment; mirth;
   gayety, with laughter.
   [1913 Webster]

WordNet (r) 3.0 (2006):

merriness
    n 1: the trait of merry joking [syn: jocoseness, jocosity,
         merriness, humorousness]