On the off chance:

碰碰運(yùn)氣

例句：

I didn't think you'd be at home but I just called by on the off chance. 

我想你不會(huì)在家，只是碰碰運(yùn)氣順路來(lái)看一下。

Take a chance:

碰運(yùn)氣

例句：

You take a chance on the weather if you holiday in Maine. 

如果你在緬因州度假，對(duì)天氣得碰運(yùn)氣。

Roll the dice:

擲骰子；碰碰運(yùn)氣

例句： 

She decided to roll the dice this time.

她決定這次碰碰運(yùn)氣。