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."
Proto-Indo-European root meaning "wild beast."
It is the hypothetical source of/evidence for its existence is provided by: Latin ferus "wild, untamed;" Greek thēr, Old Church Slavonic zveri, Lithuanian žvėris "wild beast."
1530s, from Latin truculentus "fierce, savage, stern, harsh, cruel," from trux (genitive trucis) "fierce, rough, savage, wild," perhaps from a suffixed form of PIE root *tere- (2) "cross over, pass through, overcome." Related: Truculently.
c. 1300, raumpaunt, "standing on the hind legs" (as a heraldic lion often does), thus, also, "fierce, ravenous" (late 14c.), from Old French rampant, rampans, present participle of ramper "to climb, scale, mount" (see ramp (v.)). Sense of "growing without check" (in running rampant), is recorded by 1610s, probably is via the notion of "fierce disposition" or else preserves the older French sense. Related: Rampantly.
figurative for a puritan attitude toward the arts at least since Shaw (1916), from the name of Girolamo Savonarola (1452-1498), Dominican monk famous for his fierce opposition to moral licence and Church corruption.