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