1590s, "a doubling," from Latin geminationem (nominative geminatio) "a doubling," noun of action from past-participle stem of geminare "to double, repeat" (see geminate). In rhetoric, repetition of a word or phrase for emphasis.

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).

1570s, "joints, connecting links," alteration of gemel "twins" (late 14c.), from Old French jumel "a twin" (12c., Modern French jumeau), from Latin gemellus, diminutive of geminus (adj.) "twin, born together" (see geminate). As a type of contrivance for securing free motion in suspension, by 1780. Related: Gimbals. Gemmels (plural) was Middle English for "twins" (late 14c.), also "Gemini," from Old French gemeles; hence also gemel ring, a double finger-ring that may be taken apart; also gimmal.

"triple, threefold," originally in anatomy, 1815, with -al (1) + Latin trigeminus "born in threes," as a noun, "triplets;" from tri- (see tri-) + geminus "born at the same birth" (see geminate (adj.)). By 1830 in reference to cranial nerves divided in three branches. Blount (1656) has trigeminous for "being one of three born together."