Etymology
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filter (v.)
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.
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filter (n.)
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 (from PIE root *pel- (5) "to thrust, strike, drive"). Figurative use from c. 1600. As a pad of absorbent material attached to a cigarette, from 1908.
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filtration (n.)
"act or process of filtering," c. 1600, perhaps from French filtration (1570s), noun of action from filter "to filter" (see filter (v.)).
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filtrate (v.)

1610s, probably a back-formation from filtration or else from Medieval Latin filtratus, from filtrum "felt" (see filter (n.)). Related: Filtrated; filtrating. As a noun, "liquid which has passed through a filter," by 1840.

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felt (n.)
unwoven fabric matted together by rolling or beating while wet, Old English felt "felt," from West Germanic *feltaz "something beaten, compressed wool" (source also of Old Saxon filt, Middle Dutch vilt, Old High German filz, German Filz, Danish filt), from Proto-Germanic *felt- "to beat," from PIE root *pel- (5) "to thrust, strike, drive," with a sense of "beating." Compare filter (n.). Felt-tipped pen (or -tip) is from 1953.
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*pel- (5)
Proto-Indo-European root meaning "to thrust, strike, drive."

It forms all or part of: anvil; appeal; catapult; compel; dispel; expel; felt (n.) "unwoven fabric matted together by rolling or beating;" filter; filtrate; impel; impulse; interpellation; interpolate; peal; pelt (v.) "to strike (with something);" polish; propel; pulsate; pulsation; pulse (n.1) "a throb, a beat;" push; rappel; repeal; repel; repousse.

It is the hypothetical source of/evidence for its existence is provided by: Greek pallein "to wield, brandish, swing," pelemizein "to shake, cause to tremble;" Latin pellere "to push, drive;" Old Church Slavonic plŭstĭ.
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coulee (n.)

"deep ravine, seasonally flooded," 1804, a North American word, originally used in areas explored by French trappers, from French coulée "flow" (17c.), from fem. past participle of couler "to flow," from Latin colare "to filter, strain" (see colander).

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percolation (n.)

"the act of straining or filtering through some porous material," 1610s, from Latin percolationem (nominative percolatio) "a straining through; the act of filtering," noun of action from past-participle stem of percolare "to strain through, filter," from per "through" (from PIE root *per- (1) "forward," hence "through") + colare "to strain," from colum "a strainer," which is of uncertain origin.

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portcullis (n.)

also port-cullis, c. 1300, port-colice, "strong grating of wood or iron made to fit in the entrance of a fortified place," from Old French porte coleice "sliding gate" (c. 1200, Modern French porte à coulisse), from porte "gate" (from Latin porta, from PIE root *per- (2) "to lead, pass over") + coleice "sliding, flowing," fem. of coleis, from Latin colatus, past participle of colare "to filter, strain," which is of uncertain origin.

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strain (v.)

c. 1300, "tie, bind, fasten, gird," from present participle stem of Old French estreindre "bind tightly, clasp, squeeze," from Latin stringere (2) "draw tight, bind tight, compress, press together," from PIE root *streig- "to stroke, rub, press" (source also of Lithuanian strėgti "congeal, freeze, become stiff;" Greek strangein "twist;" Old High German strician "mends nets;" Old English streccian "to stretch;" German stramm, Dutch stram "stiff").

From late 14c. as "tighten; make taut," also "exert oneself; overexert (a body part)," Sense of "press through a filter, put (a liquid) through a strainer" is from early 14c. (implied in strainer); that of "to stress beyond measure, carry too far, make a forced interpretation of" is from mid-15c. Related: Strained; straining.

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