On the off chance:
碰碰运气
例句：
I didn't think you'd be at home but I just called by on the off chance.
我想你不会在家，只是碰碰运气顺路来看一下。
Take a chance:
碰运气
例句：
You take a chance on the weather if you holiday in Maine.
如果你在缅因州度假，对天气得碰运气。
Roll the dice:
掷骰子；碰碰运气
例句：
She decided to roll the dice this time.
她决定这次碰碰运气。
