The Collaborative International Dictionary of English v.0.48:

Incoherence \In`co*her"ence\, Incoherency \In`co*her"en*cy\, n.
   [Cf. F. incoh['e]rence.]
   [1913 Webster]
   1. The quality or state of being incoherent; lack of
      coherence; lack of cohesion or adherence. --Boyle.
      [1913 Webster]

   2. Lack of connection; incongruity; inconsistency; lack of
      agreement or dependence of one part on another; as, the
      incoherence of arguments, facts, etc.
      [1913 Webster]

            Incoherences in matter, and suppositions without
            proofs, put handsomely together, are apt to pass for
            strong reason.                        --Locke.
      [1913 Webster]

   3. That which is incoherent.
      [1913 Webster]

            Crude incoherencies . . . and nauseous tautologies.
                                                  --South.
      [1913 Webster]

WordNet (r) 3.0 (2006):

incoherency
    n 1: lack of cohesion or clarity or organization [syn:
         incoherence, incoherency] [ant: coherence,
         coherency, cohesion, cohesiveness]
    2: nonsense that is simply incoherent and unintelligible [syn:
       incoherence, incoherency, unintelligibility]