late 14c., from Old Norse fyri- "fir" or Old Danish fyr, both from Proto-Germanic *furkhon (source also of Old High German foraha, German Föhre "fir"), from PIE root *perkwu-, originally meaning "oak," also "oak forest," but never "wood" (source also of Sanskrit paraktah "the holy fig tree," Hindi pargai "the evergreen oak," Latin quercus "oak," Lombardic fereha "a kind of oak"). Old English had a cognate form in furhwudu "pine wood" (only in glosses, for Latin pinus), but the modern English word is more likely from Scandinavian and in Middle English fyrre glosses Latin abies "fir," which is of obscure origin.
According to Indo-Europeanists Gamkrelidze and Ivanov, "The semantics of the term clearly points to a connection between 'oak' and mountainous regions, which is the basis for the ancient European term applied to forested mountains" (such as Gothic fairgunni "mountainous region," Old English firgen "mountain forest," Middle High German Virgunt "mountain forest; Sudetes"). In the period 3300 B.C.E. to 400 B.C.E., conifers and birches gradually displaced oaks in northern European forests. "Hence it is no surprise that in the early history of the Germanic languages the ancient term for mountain oak and oak forest shifts to denote conifers and coniferous forests." [Thomas V. Gamkrelidze, Vjaceslav V. Ivanov, "Indo-European and the Indo-Europeans," Berlin, 1994]