Gemini (n.)

late 14c., "a beaver," from Old French castor (13c.), from Latin castor "beaver," from Greek kastor "beaver," perhaps literally "he who excels," and thus identical with the name of one of the divine twins (with Pollux), worshipped by women in ancient Greece as a healer and preserver from disease (see Castor).

It has been assumed that the hero's name was given to the animal because he was a noted healer and the odorous reddish-brown secretions of the inguinal sacs of the animal (Latin castoreum), were used medicinally in ancient times, especially for women's diseases. But the animal did not live in Greece in classical times (the closest beavers were north of the Black Sea), and the name probably was borrowed from another language, perhaps influenced by the hero's name. The Greek word replaced the native Latin word for "beaver" (fiber).

In English, castor is attested in the secretion sense from late 14c. Modern castor oil is so-called by 1746; it is made from seeds of the plant Ricinus communis but supposedly possesses the laxative qualities (and taste) of beaver juice.

in Greek mythology, Castor and Polydeuces (Pollux), warrior-gods and tutelary protectors of sailors, twin sons of Zeus and Leda; a Latinized form of Greek Dioskouroi, literally "Zeus' boys," from Dios, genitive of Zeus (see Zeus) + kouroi, plural of kouros "boy, son," from PIE *korwo- "growing" (hence "adolescent"), from suffixed form of root *ker- (2) "to grow." Related: Dioscuric; Dioscurian.