From Latin 'ferus' (wild, untamed), from PIE *ǵʰwer- (wild animal) — French kept 'proud,' English took 'aggressively intense.'
Showing a heartfelt and powerful intensity; violently aggressive or hostile; extremely strong or severe.
From Old French 'fiers' (fierce, proud, brave, cruel), nominative form of 'fer' (wild, savage), from Latin 'ferus' (wild, untamed, savage), from PIE *gʷʰer- (wild animal, beast) — the same root that gives 'feral,' 'ferocity,' and possibly 'forest' (the place of wild things). The PIE root *gʷʰer- or *ǵʰwer- is reconstructed as the word for a wild beast or wilderness; it appears in Sanskrit 'ghora' (terrifying, awful), and its reflex in Latin 'ferus' gave a large family of English words through both direct borrowing and French. Old French shifted the meaning toward pride
Modern French 'fier' means 'proud' — preserving the Old French sense — while English 'fierce' means 'aggressively intense.' The same Latin word split into opposite emotional registers: French pride versus English aggression. Meanwhile, Greek 'θήρ' (thēr, 'wild beast'), from the same PIE root, gave English 'therianthrope' (a were-animal) through