From Longman Dictionary of Contemporary EnglishSiamese twinˌSiamese ˈtwin noun [countable usually plural] TWOone of two people who are born joined to each other – sometimes considered offensive SYN conjoined twin
Examples from the Corpus
Siamese twin• Eileen Delaney has five cats in all ... including Pickle's Siamese twin Branston.Origin Siamese twin (1800-1900) From such a pair (Chang and Eng) who were born in Siam (now Thailand) 