to secretly enter or join a group, organization, or place without permission, like a spy joining an enemy group.
From in- + filtrate (from Medieval Latin filtrare 'to filter'). The idea is to seep in gradually like liquid through a filter, concealing your entry.
Infiltration tactics are ancient—Julius Caesar wrote about spies infiltrating enemy camps, but the word 'infiltrate' wasn't commonly used until World War II when espionage became industrial-scale.
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