the.com/infiltrator

means Someone who secretly gets inside a group or organization in order to spy on it or undermine it from within.

from Built from Latin in- ('into') plus filtrare, 'to filter' or 'strain through,' the same root that gives us 'filter.' The image is of a liquid seeping slowly through small gaps until it's fully inside — 'infiltrate' first described armies or fluids quietly penetrating a barrier, and only later came to mean a person who sneaks into enemy or rival territory, often as a spy or saboteur. 'Infiltrator' is the agent-noun that followed, naming the one doing the seeping.