She had known Max for some years now, but he was still pretty much an unknown quantity.

她现在认识马克斯也好几年了，但是他仍然是个大大的未知数。

《柯林斯英汉双解大词典》

Are an unknown quantity.

对于他们来说是一个未知的组合。

youdao

In algebra, the sign X usually denotes an unknown quantity.

在代数里，符号X常用来表示未知数。

youdao

Yet as far as Channel 4 is concerned, Ahmadinejad is still an unknown quantity for most Britons.

CENTERFORSECURITYPOLICY: The ��realist�� fantasy

But in some key repertoire for the Philharmonic, Bruckner and Schumann for instance, he is an unknown quantity.

ECONOMIST: Why Berlin��s musicians don��t need a maestro

At 18, Prince Hamzeh is an unknown quantity (just as King Hussein was when he became king, aged 17).

ECONOMIST: How indispensable is King Hussein?