1911 Encyclopædia Britannica/Rein
REIN, a guiding or controlling leather strap or thong, attached to the bit of a ridden or driven horse (see Saddlery). The word is taken from the O. Fr. rene, modern rêne, and is usually traced to a supposed Late Latin substantive retina formed from retinere, to hold back, restrain, cf. classical Latin retinaculum, halter. The word, usually in the plural, has been often used figuratively, as a type of that which guides, restrains or controls, e.g. in such phrases as the “reins of government,” &c. The “reins,” i.e. the kidneys (Lat. renes, cf. Gr. φρήν, the midriff), or the place where the kidneys are situated, hence the loins, also, figuratively, the seat of the emotions or affections, must be distinguished.