The Collaborative International Dictionary of English v.0.48:

Inconceivability \In`con*ceiv`a*bil"i*ty\, n.
   The quality of being inconceivable; inconceivableness.
   [1913 Webster]

         The inconceivability of the Infinite.    --Mansel.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inconceivability
    n 1: the state of being impossible to conceive [syn:
         inconceivability, inconceivableness]