Perfidious means treacherous towards those who trusted you. Perfidious behaviour.
The betrayal of trust is the whole of it, since an enemy cannot be perfidious. Perfidious Albion was the French name for Britain and lasted three centuries.
1adjectiveperfidiousTreacherous towards those who trusted you.
“The whole scheme was perfidious and entirely legal.”
perfidious is a quality.
Latin perfidiosus comes from perfidia, treachery, from per plus fides, faith.
Latin
perfidiōsus
English, today
perfidious
Borrowed from Latin perfidiōsus. From Latin perfidiōsus (“treacherous”), from perfidia. by surface analysis, perfidy + -ous.
Etymology from Wiktionary, CC BY-SA