The Collaborative International Dictionary of English v.0.48:

Unlikelihood \Un*like"li*hood\, n.
   Absence of likelihood.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unlikelihood
    n 1: the improbability of a specified outcome [syn:
         unlikelihood, unlikeliness] [ant: likelihood,
         likeliness]