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). The extended sense of "intense nervous excitement" is from 1580s. Also as a verb in Old English, feferian.