Siamese twins (linguistics)
From The Art and Popular Culture Encyclopedia
Siamese twins (also irreversible binomials, binomials, binomial pairs, freezes) in the context of the English language refers to a pair or grouping of words that is used together as an idiomatic expression or collocation, usually conjoined by the words and or or. The order of elements cannot be reversed. The expressions hammer and sickle (two nouns), short and sweet (two adjectives), and sink or swim (two verbs) are various examples of Siamese twins.
Some English words (e.g. vim in vim and vigor or the abet in aid and abet) are found more often in such phrases than on their own. In other cases Siamese twins are catchy (and thus clichés and catchphrases) due to alliteration, rhyming, or their ubiquity in society and culture. Word combinations like rock and roll, the birds and the bees, mix and match, and wear and tear have become so widely used that their meanings surpass the meaning of the constituent words and are thus inseparable and permanent parts of the English lexicon; the former two are idioms, whilst the latter two are collocations.