Etymology
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Castor 

one of the divine twins (brother of Pollux), also the name of the alpha star of Gemini, Latin, from Greek Kastor, perhaps literally "he who excels." They were sons of Tyndarus, king of Sparta (but in post-Homeric myth of Zeus in the form of a swan)  and Leda.

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castor (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.

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Pollux 

twin brother of Castor (q.v.), hence also the name of the beta star of Gemini (though slightly brighter than Castor), 1520s, from Latin, from Greek Polydeukēs, literally "very sweet," or "much sweet wine," from polys "much" (from PIE root *pele- (1) "to fill") + deukēs "sweet" (prom PIE *dleuk-; see glucose). The contraction of the name in Latin is perhaps via Etruscan [Klein].

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Gemini (n.)

zodiac constellation, late Old English, from Latin gemini (plural of adjective geminus) "twins" (see geminate). Formerly also spelled gemeny, gemony, jeminy, etc. The twins are Castor and Pollux in Latin, which also are the names of the two brightest stars in the constellation; for their Greek name see Dioscuri. Meaning "a person born under the sign of Gemini" is recorded from 1894. As an oath, from 1660s (also found in Dutch and German), perhaps a corruption of Jesu (compare jiminy).

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beaver (n.1)

"large amphibious quadruped rodent of the genus Castor," Old English beofor, befer (earlier bebr), from Proto-Germanic *bebruz (source also of Old Saxon bibar, Old Norse bjorr, Middle Dutch and Dutch bever, Low German bever, Old High German bibar, German Biber), from PIE *bhebhrus, reduplication of root *bher- (2) "bright; brown" (source also of Lithuanian bebrus, Czech bobr, Welsh befer). See bear (n.) for the proposed reason for this.

The animal formerly was valued and hunted for its secretions (see castor) and for its fur, which was used in the manufacture of hats, so much so that beaver could mean "hat" from 1520s and continued so into 19c. even after hats began to be made of silk or other material. They were hunted to extinction in Great Britain in the 16th century but have lately been reintroduced.

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ricin (n.)

poison obtained from the castor-oil bean, 1888, from ricinus, genus name of the castor-oil plant (1694), from Latin ricinus (Pliny), a word of uncertain origin, perhaps the same word as ricinus "tick" (in sheep, dogs, etc.). Latin ricinum was used in late Old English herbariums.

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Leda 

in Greek mythology, wife of Tyndareus, a king of Sparta; she was mother of Clytaemnestra, Helen, Castor, and Pollux.

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caster (n.2)

"small wheel and swivel attached to the leg of a piece of furniture," 1748, agent noun from cast (v.) in the old sense of "turn." Also sometimes castor.

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caster (n.1)

also sometimes castor, "person or thing that casts," late 14c., agent noun from cast (v.). The meaning "pepper shaker, small perforated container" is from 1670s, on the notion of something that "throws" the powder, liquid, etc., when needed.

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Dioscuri 

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.

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