per ‧ fid ‧ i ‧ ous /pəˈfɪdiəs $ pər-/ BrE AmE adjective literary
[ Date: 1500-1600 ; Language: Latin ; Origin: perfidiosus , from perfidia 'disloyalty' , from perfidus 'disloyal' , from per fidem decipere 'to deceive by trust' ]
someone who is perfidious is not loyal and cannot be trusted SYN treacherous