likely to change suddenly and without reason and therefore not able to be predicted (= expected before it happens) or depended on

The weather there can be unpredictable - one minute it's blue skies and the next minute it's pouring rain. 那儿的天气有点儿变幻莫测—— 一会儿晴空万里，一会儿又大雨倾盆。

The hours in this job are very unpredictable - you sometimes have to work late at very short notice. 这份工作的时间非常不固定——有时突然接到通知就得工作到很晚。