From Longman Dictionary of Contemporary Englishperfidiousper‧fid‧i‧ous /pəˈfɪdiəs $ pər-/ adjective literary TRICK/DECEIVEsomeone who is perfidious is not loyal and cannot be trusted SYN treacherous
Examples from the Corpus
perfidious• The air was thick with paranoia as the conversation turned to the perfidious question of appearance money.• a perfidious schemeOrigin perfidious (1500-1600) Latin perfidiosus, from perfidia “disloyalty”, from perfidus “disloyal”, from per fidem decipere “to deceive by trust” 