The Collaborative International Dictionary of English v.0.48:

Unfaithful \Un*faith"ful\, a.
   1. Not faithful; not observant of promises, vows, allegiance,
      or duty; violating trust or confidence; treacherous;
      perfidious; as, an unfaithful subject; an unfaithful agent
      or servant.
      [1913 Webster]

            My feet, through wine, unfaithful to their weight.
                                                  --Pope.
      [1913 Webster]

            His honor rooted in dishonor stood,
            And faith unfaithful kept him falsely true.
                                                  --Tennyson.
      [1913 Webster]

   2. Not possessing faith; infidel. [R.] --Milton.
      [1913 Webster] -- Un*faith"ful*ly, adv. --
      Un*faith"ful*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unfaithfulness
    n 1: the quality of being unfaithful [syn: infidelity,
         unfaithfulness] [ant: faithfulness, fidelity]