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A female animal, usually a calf, that is born as the twin of a male animal and is sterile because of having abnormal internal reproductive organs.
[Probably free-, of unknown origin + an element akin to Scots mart, ox or cow fattened for slaughter (as freemartins often are) (from Scottish Gaelic, from Old Irish), perhaps influenced by the name of St. Martin of Tours, because cattle were traditionally slaughtered and salted for winter use on Martinmas.]
(Veterinary Science) the female of a pair of twin calves of unlike sex that is imperfectly developed and sterile, probably due to the influence of the male hormones of its twin during development in the uterus
[C17: of uncertain origin]
a female calf that is born as a twin with a male and is sterile as a result of exposure to masculinizing hormones produced by the male.
[1675–85; orig. uncertain]