'infiltrator' originates from Latin, specifically the word 'infiltrare,' where 'in-' meant 'into' and 'filtrare' meant 'to filter.'

'infiltrare' transformed into the French word 'infiltrer,' and eventually became the modern English word 'infiltrate' through Middle English.

Initially, it meant 'to filter into,' but over time it evolved into its current meaning of 'to enter or join secretly.'