per·fid·i·ous \pə(r)ˈfidēəs\adjective Etymology: Latin perfidiosus, from perfidia perfidy + -osus -ous — more at perfidy : of, relating to, or characterized by perfidy : deceitful, treacherous < that common but most perfidious refuge of men of letters … the profession of teaching — Matthew Arnold > Synonyms:see faithless