1.91 USAGE NOTE

In some cases, the meaning of a compound noun is not obvious from the words it consists of.

For example, someone's 'mother-tongue' is not the tongue of their mother but the language they learn as a child, and an 'old hand' is not a hand which is old but a person who is experienced at doing a particular job.

In other cases, the compound noun consists of words which do not occur on their own, for example 'hanky-panky', 'hodge-podge', and 'argy-bargy'. These nouns are usually used in informal conversation rather than formal writing.