Why are twins called Siamese twins?

Chang and Eng Bunker (no relation to Archie) were the first pair to survive to adulthood. They were male and they hailed from Siam, hence the name. It is a congenital anomaly and is not linked to any specific ethnic group. The Hilton sisters appeared to be partly at least Italo-American. Conjoined twins is the preferred term. Siam (now Thailand) rejected the term as racist and asked other countries to not use that term.