mid-13c., "proud, noble, bold," from Old French fers, nominative form of fer, fier "strong, overwhelming, violent, fierce, wild; proud, mighty, great, impressive" (Modern French fier "proud, haughty"), from Latin ferus "wild, untamed," from PIE root *ghwer- "wild, wild animal" (cf. Greek ther, Old Church Slavonic zveri, Lithuanian zveris "wild beast").
Original English sense of "brave, proud" died out 16c., but caused the word at first to be commonly used as an epithet, which accounts for the rare instance of a French word entering English in the nominative case. Meaning "ferocious, wild, savage" is from c.1300. Related: Fiercely; fierceness.