Infiltrate

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Middle English (as infiltration ): from in-2 + filtrate.

wiktionary

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From Middle English infiltrate(adj), from Medieval Latin infiltrātus, from infiltrō.

etymonline

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infiltrate (v.)

1758, of fluids, from in- (2) "in" + filtrate (v.). Perhaps modeled on French infiltrer (16c.). Military sense of "penetrate enemy lines" attested from 1934. Related: Infiltrated; infiltrating.