Source : Webster's Revised Unabridged Dictionary (1913)

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.

            My feet, through wine, unfaithful to their weight.
                                                  --Pope.

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

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

Source : WordNet®

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: deliberately and abominably disloyal or likely to betray
        trust or confidence; "the faithless Benedict Arnold"; "a
        lying traitorous insurrectionist" [syn: {faithless}, {traitorous}]
     4: not trustworthy; "an unfaithful reproduction"