rainy

Fig. to reserve something--usually money--for some future need. I've saved a little money for a rainy day. Keep some extra allowance for a rainy day.

to keep something, esp. money, for a time in the future when it might be needed It looks like people may be saving a little more for a rainy day.

to keep an amount of money for a time in the future when it might be needed She has a couple of thousand pounds kept aside which she's saving for a rainy day.

A time of need or trouble, as in We knew a rainy day would come sooner or later. This idiom is often used in the context of save for a rainy day, which means to put something aside for a future time of need. [Late 1500s]

Provide contingency funds for when times are tough. There's no clear answer to when this expression began (some have traced it back to the 16th century), but it's clear that a “rainy day” is the symbol of gloom. The wise course, therefore, is to sock away funds to tide you over when times are tough.