adjective

having or displaying an intense or ferocious aggressiveness.

fierce fighting continued throughout the day

ferocious,savage,vicious,wild,feral,untamed,undomesticated,aggressive,bloodthirsty,dangerous,cruel,brutal,murderous,menacing,threatening,terrible,grim,cut-throat,competitive,keen,intense,strong,relentless

adverb

very; extremely.

he was fierce proud