unpredictable
adj
(a) that cannot be predicted 無法預料的; 不可預知的
an unpredictable result 難以預料的結果.
(b) (of a person) whose behaviour cannot be predicted; changeable; unstable(指人)行為難以預知的, 易變的, 不穩定的
You never know how she'll react
she's so unpredictable. 誰也不知道她反應如何, 她這個人反覆無常.