Falling or flowing downward in large quantities, like rain or water pouring down (present participle form).
From 'down' plus 'pour' (from Old French 'porer', to push or thrust). The compound 'downpour' as a noun for heavy rain dates to the 18th century; the verb form follows naturally.
A 'downpour' is one of those wonderfully expressive English words that sounds like what it means—the repetition of the 'ow' sounds makes you almost feel the water falling! It's called onomatopoeia.
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