fierceyoudaoicibaDictYouDict[fierce 词源字典]
fierce: [13] Fierce has not always had exclusively negative connotations of ‘aggression’, although admittedly they do go back a long way. Its source, Latin ferus (which also gave English feral) meant originally ‘wild, untamed’, but it subsequently developed the metaphorical sense ‘uncultivated, savage, cruel’. However, when English acquired the word, via Anglo-Norman fers and Old French fiers, it was used for ‘brave’ and ‘proud’ as well as ‘wildly hostile or menacing’. ‘Brave’ died out in the 16th century, although across the Channel ‘proud’ has survived to become the only sense of modern French fiers.
=> feral[fierce etymology, fierce origin, 英语词源]
fierce (adj.)youdaoicibaDictYouDict
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; wild animal" (cognates: Greek ther, Old Church Slavonic zveri, Lithuanian zveris "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."