Perfidious /(pẽr*fĭd"ĭ*ŭs; 277)/

Per·fid·i·ous

Perfidious

a.
  1. Guilty of perfidy; violating good faith or vows; false to trust or confidence reposed; treacherous; faithless; as, a perfidious friend.
  2. Involving, or characterized by, perfidy.