Tā suīrán wàibiǎo róuruò, dàn yù shì què shífēn xiōnghàn.
Although she appears gentle on the outside, she becomes ferociously tough when faced with a challenge.
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usage
凶悍 describes something fiercely aggressive and difficult to subdue. It can describe people, animals, or fighting styles. Compare 凶猛 (ferocious/fierce — more about raw power) and 强悍 (strong and tough — often positive).