Secretly entered or joined a group or place, often to gather information or cause harm.
From 'in-' (in) and 'filtrate' (to pass through). The word originally meant 'to pass through gradually,' then evolved to mean sneaking in secretly.
Spies and undercover agents have used infiltration for centuries, but the word itself is quite recent—it became common in the 1800s when secret police forces started systematically infiltrating revolutionary groups.
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