The Collaborative International Dictionary of English v.0.48:

Unreliable \Un`re*li"a*ble\, a.
   Not reliable; untrustworthy. See Reliable. --
   Un`re*li"a*ble*ness, n.
   [1913 Webster]

         Alcibiades . . . was too unsteady, and (according to
         Mr. Coleridge's coinage) "unreliable;" or perhaps, in
         more correct English, too "unrelyuponable." --De
                                                  Quincey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unreliableness
    n 1: the trait of not being dependable or reliable [syn:
         undependability, undependableness, unreliability,
         unreliableness] [ant: dependability, dependableness,
         reliability, reliableness]