Slowly and secretly entering or becoming part of a group, organization, or place without being noticed.
From 'infiltrate' (in- + filtrate, meaning to filter or seep through). From Latin 'filtrare' (to strain through). The word entered English in the 1700s with the sense of liquid seeping through, then was applied to spies and secret movements.
The word 'infiltrate' is perfect because water infiltrating soil literally seeps through without making a huge entrance—the same way a spy might move! This natural metaphor made the word ideal for describing secret military and espionage operations starting in the 1800s.
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