WordWeb Online

1. Marked by extreme and violent energy
   "fierce fighting"
   - ferocious, furious, savage
    
2. Marked by extreme intensity of emotions or convictions; inclined to react violently; fervid
   "fierce loyalty"
   - tearing, vehement, violent, trigger-happy
    
3. Ruthless in competition
   "fierce competition"
   - cutthroat, bowelless
    
4. Violently agitated and turbulent
   "the fierce thunders roar me their music"
   - boisterous, rough

Derived forms: fiercer, fiercest

See also: fiercely, fierceness, intense, merciless, stormy, unmerciful, violent

Encyclopedia: Fierce Fierce, Isakowitz and Blalock