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Douglas Harper's Etymology Dictionary
flood-gate

early 13c. in the figurative sense "opportunity for a great venting" (especially with reference to tears or rain); literal sense is mid-15c. (gate designed to let water in or keep it out as desired, especially the lower gate of a lock); from flood (n.) + gate (n.).

Wiktionary
flood-gate

n. (alternative spelling of floodgate English)

Usage examples of "flood-gate".

The artificial lake of Moeris was created as a reservoir for the waters of the Nile: it was 'four hundred and fifty miles in circumference' and three hundred and fifty feet deep, with subterranean channels, flood-gates, locks, and dams, by which the wilderness was redeemed from sterility.

The report that instantly followed made the White House dance upon its foundations, and, as if that had been a signal, the flood-gates of the sky immediately opened, and rain so dense that it looked like a solid cataract of water poured down upon the earth.

But, in the midst of it, the heavens opened their flood-gates, as they had done in the New World, and then the aeros, flooded with rain, and hurled about by contending blasts of wind, drooped, fluttered, and fell by hundreds into the fast mounting waves.