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]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "unlikelihood":
   bare possibility, dark horse, doubtfulness, dubiousness,
   faint likelihood, fighting chance, gambling chance,
   hardly a chance, hundred-to-one shot, implausibility,
   improbability, incredibility, little chance, little expectation,
   little opportunity, long odds, long shot, off chance,
   outside chance, poor bet, poor lookout, poor possibility,
   poor prospect, questionableness, remote possibility, slim chance,
   small chance