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.

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]

"that which reduces fever," 1680s, from anti- + Greek pyretos "fever, burning heat," related to pyr "fire" (from PIE root *paewr- "fire") + -ic. As an adjective, "reducing fever," 1837.

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.

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

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.

c. 1300, "acute fever," also (late 14c.) "malarial fever (involving episodes of chills and shivering)" from Old French ague "acute fever," from Medieval Latin (febris) acuta "sharp (fever)," from fem. of acutus "sharp" (from PIE root *ak- "be sharp, rise (out) to a point, pierce").

"fever, a higher bodily temperature than is normal," 1769, medical Latin, from Greek pyrexis "feverishness," from pyressein "to be feverish, to be ill of fever," from pyretos "fever, burning heat" (related to pyr "fire," from PIE root *paewr- "fire") + abstract noun ending -ia. Formerly sometimes nativized as pyrexy. Related: Pyrexial; pyrexic; pyrexical.

"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.)).

also anti-febrile, 1660s, "having the property of abating fever," from anti- + febrile. As a noun, "substance which abates fever," 1859.