Etymology
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fever (n.)

earlier also feaver, late Old English fefor, fefer "fever, temperature of the body higher than normal," from Latin febris "fever," related to fovere "to warm, heat," which is probably from PIE root *dhegh- "burn" (source also of Gothic dags, Old English dæg "day," originally "the heat;" Greek tephra "ashes;" Lithuanian dāgas "heat," Old Prussian dagis "summer;" Middle Irish daig "fire"); but some suggest a reduplication of a root represented by Sanskrit *bhur- "to be restless."

The Latin word was adopted into most of the Germanic languages (German Fieber, Swedish feber, Danish feber), but not Dutch. English spelling was influenced by Old French fievre.

An alternative word for "fever" was Old English hrið, hriðing (which is cognate with Old High German hritto, Irish crith, Welsh cryd, Lithuanian skriečiù, skriesti); Latin febris also was glossed by bryneadl. The extended sense of "intense nervous excitement" is from 1580s. Also as a verb in Old English, feferian.

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hay fever (n.)

also hay-fever, 1825, from hay + fever. Also called summer catarrh (1828); not much noted before the 1820s, when it was sometimes derided as a "fashion" in disease.

People are apt to sneeze, in hot weather for example; and people do not die of sneezing now-a-days, as they did in days that no one knows any thing about. We cannot give six draughts a-day, at one and nine pence each, for sneezing: call it the hay-fever. What a wonderful man! what a clever man! he understands the hay-fever: call him in! Thus is the hay-fever among the last in the list of fashionables. ["On Fashions in Physic," London Magazine, October 1825]
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febrile (adj.)

1650s, from Medieval Latin febrilis "pertaining to fever," from Latin febris "a fever" (see fever).

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feverous (adj.)

late 14c., "having a fever; characteristic of fever," from fever + -ous or from Old French fievrous. Meaning "apt to cause fever" is from 1620s. Related: Feverously.

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feverish (adj.)

late 14c., "causing fever;" 1630s, "excited, unduly ardent;" 1640s, "having symptoms of fever, having a slight fever," from fever + -ish. Earlier in same sense was feverous (late 14c.). Old English had feferig, feferseoc. Related: Feverishly; feverishness.

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febrifuge (n.)

"medicine that reduces fever," 1680s, from French fébrifuge, literally "driving fever away," from Latin febris (see fever) + fugare "cause to flee, put to flight, drive off, chase away, rout," also used in reference to banishment and exile, derived verb from fuga "flight," from PIE *bhug-a-, suffixed form of root *bheug- (1) "to flee" (see fugitive (adj.)).

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fomites (n.)

"inanimate objects that, when contaminated with or exposed to infectious agents, can retain and transfer the disease," plural of fomes (1650s), which is from medical Latin fomes (used in this sense first by Fracastoro, 16c., probably on the notion of "fuel"), from Latin fomes, fomitis "kindling-wood, touchwood, tinder," from fovere "to warm, keep warm" (see fever). The classically incorrect back-formed singular fomite is attested from 1859.

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foment (v.)

early 15c., "apply hot liquids," from Old French fomenter "apply hot compress (to a wound)" (13c.), from Late Latin fomentare, from Latin fomentum "warm application, poultice," contraction of *fovimentum, from fovere "to warm; cherish, encourage" (see fever). Extended sense of "stimulate, instigate" (1620s), on the notion of "encourage the growth of," as if by heat, probably was taken from French. Related: Fomented; fomenting.

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day (n.)

Old English dæg "period during which the sun is above the horizon," also "lifetime, definite time of existence," from Proto-Germanic *dages- "day" (source also of Old Saxon, Middle Dutch, Dutch dag, Old Frisian di, dei, Old High German tag, German Tag, Old Norse dagr, Gothic dags), according to Watkins, from PIE root *agh- "a day."  He adds that the Germanic initial d- is "of obscure origin." But Boutkan says it is from PIE root *dhegh- "to burn" (see fever). Not considered to be related to Latin dies (which is from PIE root *dyeu- "to shine").

Meaning originally, in English, "the daylight hours;" it expanded to mean "the 24-hour period" in late Anglo-Saxon times. The day formerly began at sunset, hence Old English Wodnesniht was what we would call "Tuesday night." Names of the weekdays were not regularly capitalized in English until 17c.

From late 12c. as "a time period as distinguished from other time periods." Day-by-day "daily" is from late 14c.; all day "all the time" is from late 14c.  Day off "day away from work" is attested from 1883; day-tripper first recorded 1897. The days in nowadays, etc. is a relic of the Old English and Middle English use of the adverbial genitive.

All in a day's work "something unusual taken as routine" is by 1820. The nostalgic those were the days is attested by 1907. That'll be the day, expressing mild doubt following some boast or claim, is by 1941. To call it a day "stop working" is by 1919; earlier call it a half-day (1838). One of these days "at some day in the near future" is from late 15c. One of those days "a day of misfortune" is by 1936.

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feverfew (n.)

Old English feferfuge, from Late Latin febrifugia, from Latin febris "fever" + fugare "put to flight" (see febrifuge). So called for its medical usage. The modern English word probably is reborrowed from an Anglo-French source.

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