Bad things or problems tend to happen all at once rather than being spread out over time.
This phrase originated in the early 18th century, with the earliest recorded use around 1726. It was later popularized by Morton Salt Company's advertising slogan 'When it rains, it pours' in 1914, referring to their salt's ability to pour freely even in humid conditions. The metaphorical meaning predates the commercial use.
While often used for negative situations, this phrase can also apply to positive events clustering together - like getting multiple job offers at once after a long search. The meteorological metaphor is particularly apt because actual rain storms do tend to bring heavy downpours rather than gentle, steady drizzles.
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