infiltrate

Where does the word "infiltrate" come from? It comes from Medieval Latin filtrum, and was adopted directly into English (1750s).

Infiltrate’s story begins in Medieval Latin: filtrum meant “felt used as a strainer.” Felt was used to strain liquids; the source of 'filter', which English turned into 'infiltrate'. It reached English by 1750s as infiltrate. Military 'infiltration' dates from the 1900s. Today, infiltrate means to secretly enter an organization.

1. Stage 1 · Medieval Latin · medieval filtrum "felt used as a strainer", felt was used to strain liquids; the source of 'filter', which English turned into 'infiltrate'.
2. Stage 2 · English · 1750s infiltrate "to pass through a filter", military 'infiltration' dates from the 1900s.