Etymology
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lethargy (n.)

late 14c., litarge, "state of prolonged torpor or inactivity, inertness of body or mind," from Medieval Latin litargia, from Late Latin lethargia, from Greek lēthargia "forgetfulness," from lēthargos "forgetful," apparently etymologically "inactive through forgetfulness," from lēthē "a forgetting, forgetfulness" (see latent) + argos "idle" (see argon). The form with -th- is from 1590s in English. The Medieval Latin word also is the source of Old French litargie (Modern French léthargie), Spanish and Italian letargia.

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lethargic (adj.)
late 14c., litargik, "morbidly drowsy, manifesting lethargy," from Latin lethargicus "affected with lethargy," from Greek lethargikos "drowsy," from lethargos "forgetful; inactive" (see lethargy). From 1590s as "pertaining to lethargy." Related: Lethargically. In 17c. also with a verb form, lethargize, and a noun, letharge "lethargic patient."
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*werg- 
Proto-Indo-European root meaning "to do."

It forms all or part of: allergic; allergy; argon; boulevard; bulwark; cholinergic; demiurge; dramaturge; energy; erg (n.1) "unit of energy;" ergative; ergonomics; ergophobia; George; georgic; handiwork; irk; lethargic; lethargy; liturgy; metallurgy; organ; organelle; organic; organism; organize; orgy; surgeon; surgery; synergism; synergy; thaumaturge; work; wright; wrought; zymurgy.

It is the hypothetical source of/evidence for its existence is provided by: Greek ergon "work," orgia "religious performances;" Armenian gorc "work;" Avestan vareza "work, activity;" Gothic waurkjan, Old English wyrcan "to work," Old English weorc "deed, action, something done;" Old Norse yrka "work, take effect."
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sopor (n.)
Latin, "deep sleep, lethargy," from PIE *swep-os-, suffixed form of root *swep- "to sleep."
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torpor (n.)

"lethargy, listlessness," c. 1600, from Latin torpor "numbness, sluggishness," from torpere "be numb, be inactive, be dull" (from PIE root *ster- (1) "stiff").

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slow (adj.)
Old English slaw "inactive, sluggish, torpid, lazy," also "not clever," from Proto-Germanic *slæwaz (source also of Old Saxon sleu "blunt, dull," Middle Dutch slee, Dutch sleeuw "sour, tart, blunt," Old High German sleo "blunt, dull," Old Norse sljor, Danish sløv, Swedish slö "blunt, dull"). Meaning "taking a long time" is attested from early 13c. Meaning "dull, tedious" is from 1841. As an adverb c. 1500. The slows "imaginary disease to account for lethargy" is from 1843.
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phlegm (n.)

late 14c., fleem, fleume, "viscid mucus, discharge from a mucous membrane of the body," also the name of one of the four bodily humors, from Old French fleume (13c., Modern French flegme), from Late Latin phlegma, one of the four humors of the body, from Greek phlegma "morbid, clammy bodily humor caused by heat;" literally "inflammation, flame, fire, heat," from phlegein "to burn," related to phlox (genitive phlogos) "flame, blaze," from PIE *bhleg- "to shine, flash," from root *bhel- (1) "to shine, flash, burn."

The modern form of the word is attested by c. 1660. In old physiology it was the "cold, moist" humor of the body and a predominance of it was believed to cause dullness, lethargy, and apathy, hence phlegmatic.

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