filter: [14] Ultimately, filter is the same word as felt – and indeed that is what it first meant in English (‘They dwell all in tents made of black filter’, John Mandeville, Travels 1400). It comes via Old French filtre from medieval Latin filtrum, which was borrowed from prehistoric West Germanic *filtiz, source of English felt. The modern sense of filter did not develop until the 17th century; it came from the use of felt for removing impurities from liquid. The derivative infiltrate dates from the 18th century. (The homophonic philtre [16] is not related; it comes ultimately from Greek phílos ‘beloved’.) => felt, infiltrate
early 15c., "piece of felt through which liquid is strained," from Old French feutre "felt, felt hat, carpet" (Modern French filtre) and directly from Medieval Latin filtrum "felt" (used to strain impurities from liquid), from West Germanic *filtiz (see felt (n.)). Figurative use from c. 1600. As a pad of absorbent material attached to a cigarette, from 1908.
1570s (transitive), from French filtrer or from Medieval Latin filtrare, from filtrum "felt" (see filter (n.)). The figurative sense is from 1830. Intransitive use from 1798. Related: Filtered; filtering.
