Source : Webster's Revised Unabridged Dictionary (1913)

Untruthful \Un*truth"ful\, a.
   Not truthful; unveracious; contrary to the truth or the fact.
   -- {Un*truth"ful*ly}, adv. -- {Un*truth"ful*ness}, n.

Source : WordNet®

untruthfulness
     n : the quality of being untruthful [ant: {truthfulness}]