I had heard, a number of years ago, that there is a name for an type of idiomatic expression in which two things are joined to refer to one thing.
An example of this would be “raining cats and dogs”. This does not mean that cats is a metaphor for one type of rain, and dogs another, but rather that the combined terms form a joint expression with a single referent.
Likewise, “bread and butter” could be used similarly, e.g., “they should waste their time trying to sell frozen steaks; jewelry is QVC’s bread and butter”.
Is there a name for the specific type of idiomatic expression?
Best Answer
The generic term collocation includes idiomatically paired nouns, which both OP's examples are. So I'd say they are collocated pairs.
Other idiomatic usages such as strong tea and powerful computers are also collocations. In those cases, competent speakers recognise powerful tea and strong computers as "wrong", primarily because we're used to the other versions. Grammar and meaning aren't really relevant - it's just a matter of what people happen to say.
It's also worth noting that we never say it's raining dogs and cats. But as this NGram shows, we actually use that sequence more often than cats and dogs when we're not talking about the weather. Word sequence is one of the attributes of a collocation, though - coach and horses only occurs in that order.