mad money 备用金，为玩乐准备的钱，俗称过河钱。

【例句】
Whenever we travel, my wife and I keep a small purse of mad money with us just in case we find ourselves in a difficult situation.
当我和妻子旅游的时候，总会准备一些备用金，以备不时之需。

We've put away a few thousand dollars in mad money for our next vacation.
为了下个假期，我们准备了几千美元的私房钱。