"heavy or continuous shower," 1811, from the verbal phrase, from down (adv.) + pour (v.).
Entries linking to downpour
down adv.
"in a descending direction, from a higher to a lower place, degree, or condition," late Old English shortened form of Old English ofdune "downwards," originally of dune "off from (the) hill," from dune "from the hill," dative of dun "hill" (see down (n.2)). The "hill" word is general in Germanic, but this sense development is peculiar to English. As a preposition, "in a descending direction upon or along," from late 14c.
To be down on "express disapproval of" is by 1851. Down home is from 1828 as "in one's home region," as an adjective phrase meaning "unpretentious" by 1931, American English. Down the hatch as a toast is from 1931. Down to the wire is 1901, from horse-racing.
Down Under "Australia and New Zealand" attested from 1886; Down East "Maine" is from 1825; Down South "in the Southern states of the U.S." is attested by 1834. Down the road "in the future" is by 1964, U.S. colloquial. Down-to-earth "everyday, ordinary, realistic" is by 1932.
pour v.
"to cause (liquid or granular substance) to flow or stream either out of a vessel or into one," c. 1300, of unknown origin. Not in Old English; perhaps from Old French (Flanders dialect) purer "to sift (grain), pour out (water)," from Latin purare "to purify," from purus "pure" (see pure). Replaced Old English geotan. Intransitive sense of "to flow, issue forth in a stream" is from 1530s. Related: Poured; pouring; pourable. As a noun from 1790, "a pouring stream."