why are conjoined twins called siamese twins?

Siamese twins refers to famous conjoined twins Chang and Eng Bunker who were born in Thailand, formerly Siam, in 1811. The two were joined at the sternum and their livers were fused. The Bunkers traveled around the world in circus shows before marrying two sisters and settling in Wilkes County, North Carolina as ranchers and business men. Between them they had 21 children. Both men died in January 1874.

22-year-old conjoined twins Abby and Brittany Hensel have a new reality show on TLC called Abby and Brittany that premieres on August 28, 2012. The women are dicephalic paragus twins. Each has a separate heart, spine, spinal cord, lungs, and stomach and is responsible for coordinating one side of the body. They share two arms, two legs, one body, and the same liver. Rumor has it that one of them is engaged!