Source : Webster's Revised Unabridged Dictionary (1913)

Faithless \Faith"less\, a.
   1. Not believing; not giving credit.

            Be not faithless, but believing.      --John xx. 27.

   2. Not believing on God or religion; specifically, not
      believing in the Christian religion. --Shak.

   3. Not observant of promises or covenants.

   4. Not true to allegiance, duty, or vows; perfidious;
      trecherous; disloyal; not of true fidelity; inconstant, as
      a husband or a wife.

            A most unnatural and faithless service. --Shak.

   5. Serving to disappoint or deceive; delusive; unsatisfying.
      ``Yonder faithless phantom.'' --Goldsmith. --
      {Faith"less*ly}, adv.{Faith"less*ness}, n.

Source : WordNet®

faithlessness
     n : unfaithfulness by virtue of being unreliable or treacherous
         [syn: {falseness}, {fickleness}, {inconstancy}]