The Collaborative International Dictionary of English v.0.48:

Faithless \Faith"less\, a.
   1. Not believing; not giving credit.
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            Be not faithless, but believing.      --John xx. 27.
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   2. Not believing on God or religion; specifically, not
      believing in the Christian religion. --Shak.
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   3. Not observant of promises or covenants.
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   4. Not true to allegiance, duty, or vows; perfidious;
      trecherous; disloyal; not of true fidelity; inconstant, as
      a husband or a wife.
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            A most unnatural and faithless service. --Shak.
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   5. Serving to disappoint or deceive; delusive; unsatisfying.
      "Yonder faithless phantom." --Goldsmith. --
      Faith"less*ly, adv.Faith"less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

faithlessly
    adv 1: in a disloyal and faithless manner; "he behaved
           treacherously"; "his wife played him false" [syn:
           faithlessly, traitorously, treacherously,
           treasonably, false]