before vowels pyr-, word-forming element form meaning "fire," from Greek pyr (genitive pyros) "fire, funeral fire," also symbolic of terrible things, rages, "rarely as an image of warmth and comfort" [Liddell & Scott], from PIE root *paewr- "fire." Pyriphlegethon, literally "fire-blazing," was one of the rivers of Hell.
Old English Genesis, first book of the Pentateuch, which tells among other things of the creation of the world, from Latin genesis "generation, nativity," in Late Latin taken as the title of first book of the Old Testament, from Greek genesis "origin, creation, generation," from gignesthai "to be born," related to genos "race, birth, descent" (from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups).
Greek translators used the word as the title of the biblical book, rendering Hebrew bereshith, literally "in the beginning," which was the first word of the text, taken as its title. Extended sense of "origin, creation" first recorded in English c. 1600.
1858, as a proposed word for "electricity considered as a material substance possessing weight," from pyro- + -gen. Meaning "fever-producer, substance which, introduced into the blood, induces fever" is from 1896. Related: Pyrogenic. Greek pyrogenes meant "born in fire, wrought by fire" (compare pyrogenesis).