unknown quantity

n. 难预测的人, 难预料的事

例句与用法：

The new teacher is still very much an unknown quantity but we have hopes that he will be a valuable member of our team.

新教师是谁现在还难以预料，但我们希望他会成为我们队里的重要一员。

The new sales director is still a bit of an unknown quantity.

新来的销售部主任大家还不大了解。

详细释义：

难以预料的人

The new teacher is still very much an unknown quantity but we have hopes that he will be a valuable member of our team.

新教师是谁现在还难以预料, 但我们希望他会成为我们队里的重要一员。