hold (something) back for a rainy day
To reserve something, especially money, for use in a time or period of unforeseen difficulty, trouble, or need. I know you want to buy a new TV with your bonus, but you should really hold that money back for a rainy day. I hold a portion of my wages back each month for a rainy day. You will inevitably be faced with unforeseen emergencies in life, that's why you have to hold some money back for a rainy day.
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