1. 
2. 
3. 
[syn: faithless, traitorous, unfaithful, treasonable, treasonous]
4.  not trustworthy; 
- Example: "an unfaithful reproduction"
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):
unfaithful
    adj 1: not true to duty or obligation or promises; "an
           unfaithful lover" [ant: faithful]
    2: having sexual relations with someone other than your husband
       or wife, or your boyfriend or girlfriend; "her husband was
       unfaithful" [ant: faithful]
    3: having the character of, or characteristic of, a traitor;
       "the faithless Benedict Arnold"; "a lying traitorous
       insurrectionist" [syn: faithless, traitorous,
       unfaithful, treasonable, treasonous]
    4: not trustworthy; "an unfaithful reproduction"
Moby Thesaurus II by Grady Ward, 1.0:
28 Moby Thesaurus words for "unfaithful":
   derelict, disaffected, disloyal, disregardful, faithless, false,
   fickle, inattentive, inconstant, inobservant, negligent,
   nonadherent, noncompliant, nonconforming, nonobservant,
   not true to, of bad faith, perfidious, recreant, traitorous,
   treacherous, trothless, uncompliant, unconforming, unloyal,
   unobservant, unsteadfast, untrue