Siamese twin

plural Siamese twins

1. A conjoined twin; one of two people physically joined together.

   the two stood together; the old miser leaning against the herb-doctor with something of that air of trustful fraternity with which, when standing, the less strong of the Siamese twins habitually leans against the other.

   1857, Herman Melville, chapter XXI, in The Confidence-Man: His Masquerade

   1 more example

   The village sign on the green depicts the Biddenden Maids, the Siamese twins Elizabeth and Mary Chulkhurst, who were born about 1135, joined at the hip and shoulder. They lived until the age of 34 when they died within a few hours of each other.

   1980, AA Book of British Villages, Drive Publications Ltd, page 58

2. broadlyOne of two people who are extraordinarily close, or always found together.

   Didn't you hear what he said about your Lesta and the Prince — that they were as thick as butter — regular Siamese twins and you couldn't separate 'em?

3. human sciencesEither of a pair of words that occur together as an idiomatic expression or collocation, as in "hammer and sickle", "short and sweet", or "spick and span".