mid-13c., "proud, noble, bold, haughty," from Old French fers, fiers, nominative form of fer, fier "strong, overwhelming, violent, fierce, wild; proud, mighty, great, impressive" (Modern French fier "proud, haughty"), from Latin ferus "wild, untamed, uncultivated; waste, desert;" figuratively "wild, uncultivated, savage, cruel," from PIE root *ghwer- "wild beast."
Meaning "ferocious, wild, savage, cruel" of persons is from c. 1300; of beasts from late 14c. Original English sense of "brave, proud" died out 16c., but while this sense was current fierce often was used in English as an epithet (and thus surname), which accounts for the rare instance of a French word entering English in the nominative case. Related: Fiercely; fierceness. In Middle English sometimes also "dangerous, destructive; great, strong; huge (in number)." An early 15c. medical treatise has fers benes for "wild beans."