'perfidious' originates from Latin, specifically the word 'perfidiosus,' where 'per-' meant 'through' and 'fides' meant 'faith.'

'perfidiosus' transformed into the Old French word 'perfidie,' and eventually became the modern English word 'perfidious' through Middle English.

Initially, it meant 'faithless or treacherous,' and this meaning has largely remained the same in modern usage.