Joined at The Hip
Meaning
Inextricably linked – inseparable.
Origin
The term joined at the hip derives from situation of conjoined twins (commonly called Siamese twins), although actual conjoined twins are rarely joined at the hip.
Chang and Eng Bunker (1811-1874) were a celebrated (although not the first) pair of conjoined twins and, being from Siam, they are the source of the term Siamese twins. They were billed under that name in P.T. Barnum’s circus throughout the 1830s. They were joined at the sternum (chest). The figurative use of the term is American and dates from the 1960s. The earliest printed record of it that I can find is from the Californian newspaper the Pasadena Star-News, March 1963:
“The two organizations [Caltech and the Pasadena Chamber of Commerce] were so closely knit… they were practically joined at the hip.”