rainy day
/ˌreɪnɪ ˈdeɪ/noun
1A day characterized or dominated by rainfall; (Meteorology ) = rain day.
2 Figurative. A possible future time of hardship or relative poverty. Frequently in for (also against) a rainy day.
Origin
Late Middle English; earliest use found in John Trevisa (c1342–?1402), translator. From rainy + day.