What is the origin of the idiom "joined at the hip" and when to use it?

The origin of the phrase is not entirely clear, but it is believed to have originated in the United States in the mid-20th century. The term "joined at the hip" may have been derived from medical terminology, where doctors use the term "hip joint" to describe the place where the leg bone connects to the hip bone. The phrase gained popularity in the 1960s and 1970s and has since become a common expression used in everyday conversation to describe close friendships or relationships.