Etymology
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letting (n.)
"action of allowing movement or passage of something," early 15c., verbal noun from let (v.). Archaic or legalese meaning "delay, hindrance" is late Old English, from let (n.).
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blood-letting (n.)
also bloodletting, early 13c., blod letunge, from blood (n.) + letting. Hyphenated from 17c., one word from mid-19c. Old English had blodlæte "blood-letting," from blodlætan "to bleed, let blood."
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phlebotomy (n.)

"blood-letting," c. 1400, flebotomye, fleobotomie, from Old French flebotomie (13c., Modern French phlébotomie) and directly from Medieval Latin phlebotomia, from Greek phlebotomia "blood-letting," from phlebotomos "opening veins," from phleps (genitive phlebos) "a vein" (a word of uncertain origin) + tomē "a cutting" (from PIE root *tem- "to cut").

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farming (n.)
1590s, "action of farming out, practice of letting or leasing taxes, etc., for collection," verbal noun from farm (v.). Meaning "business of cultivating land, husbandry" is attested by 1733.
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hemophobia (n.)
1844, from hemo- "blood" + -phobia "fear." Perhaps based on French hémophobie. Originally in reference to fear of medical blood-letting.
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-lysis 
scientific/medical word-forming element meaning "loosening, dissolving, dissolution," from Greek lysis "a loosening, setting free, releasing; dissolution; means of letting loose," from lyein "to unfasten, loose, loosen, untie," from PIE root *leu- "to loosen, divide, cut apart."
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Hippolytus 
masc. proper name, son of Theseus in Greek mythology, from Greek Hippolytos, literally "letting horses loose," from hippos "horse" (from PIE root *ekwo- "horse") + stem of lyein "to unfasten, loose, loosen, untie" (from PIE root *leu- "to loosen, divide, cut apart").
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depletion (n.)

"act of emptying or reducing," 1650s, from Late Latin depletionem (nominative depletio) "blood-letting," noun of action from past-participle stem of Latin deplere "to empty," literally "to un-fill," from de "off, away" (see de-) + plere "to fill" (from PIE root *pele- (1) "to fill").

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drip (n.)

mid-15c., drippe, "a drop of liquid," from drip (v.). From 1660s as "a falling or letting fall in drops." Medical sense of "continuous slow introduction of fluid into the body" is by 1933. The slang meaning "stupid, feeble, or dull person" is by 1932, perhaps from earlier American English slang sense "nonsense" (by 1919).

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impervious (adj.)
1640s, from Latin impervius "not to be traverse, that cannot be passed through, impassible," from assimilated form of in- "not, opposite of" (see in- (1)) + pervius "letting things through, that can be passed through," from per "through" (from PIE root *per- (1) "forward," hence "through") + via "road" (see via (adv.)). Related: Imperviously; imperviousness.
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