Basic Guidelines For English Spellings
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A place or opening at which something escapes or is released downwards; especially a downward opening or channel for the passage of water or other fluids. Frequently attributive, as "downlet pipe", etc.
Chiefly in technical contexts.
Late 17th century; earliest use found in Richard Allestree (d. 1681), Church of England clergyman. From down- + let, after outlet.