"duplicated, found in pairs," early 15c., from Latin geminatus "twinned, equal," past participle of geminare "to double, repeat," related to geminus "twin, born together; paired, double," perhaps from PIE *yem- "to pair." As a verb, from 1630s. Related: Geminated; geminating; geminative.

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.

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].

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.

exclamation of surprise, 1803, colloquial form of Gemini, a disguised oath, perhaps Jesu Domine "Jesus Lord." Extended form jiminy cricket is attested from 1848, according to OED, and suggests Jesus Christ (compare also Jiminy Christmas, 1890). It was in popular use in print from c. 1901 and taken into the Pinocchio fairy tale by Disney (1940) to answer to Italian Il Grillo Parlante "the talking cricket."

also crimine, crimini, 1680s; it looks like Italian crimine "crime," but perhaps it is a deformation of Gemini (which is recorded as as an oath from 1660s) or simply another euphemism for Christ as a swear-word.