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Latin word ren comes from Proto-Indo-European *gʷʰren-
*gʷʰren- (Proto-Indo-European)
*hrēn (Proto-Italic)
ren (Latin)
(anatomy) kidney. (this term almost always plural in (ancient) Latin. See B. L. Gildersleeve's Pg. 35, Oxford Latin Dictionary Pg. 1614).