FIERCE, FEROCIOUS, BARBAROUS, SAVAGE, CRUEL mean showing fury or malignity in looks or actions.

FIERCE applies to humans and animals that inspire terror because of their wild and menacing aspect or fury in attack.

e.g. fierce warriors

FEROCIOUS implies extreme fierceness and unrestrained violence and brutality.

e.g. a ferocious dog

BARBAROUS implies a ferocity or mercilessness regarded as unworthy of civilized people.

e.g. barbarous treatment of prisoners

SAVAGE implies the absence of inhibitions restraining civilized people filled with rage, lust, or other violent passion.

e.g. a savage criminal

CRUEL implies indifference to suffering and even positive pleasure in inflicting it.

e.g. the cruel jokes of schoolboys

savagely fierce, cruel, or violent:

〈動物･人などが〉獰猛（どうもう）な, 凶暴な

e.g. the wolverine is nature's most ferocious and violent animal.

(of a conflict) characterized by or involving aggression, bitterness, and determination:

〈戦闘･議論･風雨･感情などが〉激烈［猛烈］な, 激しい

e.g. a ferocious argument.

informal extreme and unpleasant:

ものすごい, ひどい

e.g. a ferocious headache.

ferociousness |fəˈrōSHəsnəs| noun

mid 17th century: from Latin ferox, feroc- ‘fierce’+ -ious.