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: fiercest, fiercer

See also: intense, merciless, procellous [archaic], stormy, unmerciful, violent

Encyclopedia: Fierce, Isakowitz & Blalock