Perfidy descends from Latin perfidia, literally 'faith destroyed,' combining the destructive prefix per- with fidēs (faith) from PIE *bʰeydʰ- (to trust). It shares its deep root with faith, fidelity, fealty, and federal. The concept carried enormous weight in Roman public life (Cicero wielded it against Catiline) and today has a precise legal meaning as a war crime under the Geneva Conventions.
Deliberate breach of faith or trust; treachery, especially a betrayal that violates a bond of confidence or loyalty.
From Latin 'perfidia' (faithlessness, treachery, breach of trust), from 'perfidus' (faithless, treacherous), composed of 'per-' (through, thoroughly — here used as an intensifier toward destruction or completion) and 'fidēs' (faith, trust, loyalty), from PIE *bʰeydʰ- (to trust, to bind by faith). The PIE root *bʰeydʰ- is the ancestor of a dense cluster of words about trust and belief: Latin 'fidēs' gave 'fidelity, confide, diffidence, fiduciary, affidavit'; the same root in Germanic gave English 'bide' (to wait, to trust the outcome) and German 'bitten' (to ask, to entreat). 'Perfidy' encodes the completeness of the betrayal in 'per-': faith is not merely broken but destroyed
'Perfidious Albion' (la perfide Albion) has been a French epithet for England since the 17th century. Under modern international law, 'perfidy' has a precise meaning in the Geneva Conventions (Protocol I, Art. 37): acts that invite enemy confidence to betray it — such as false surrender or misusing the Red Cross — classified as a war crime.