of origin
Idiom - a group of words whose meaning
cannot be predicted from the meanings of the constituent words, or linguistic usage that is grammatical and natural to native speakers of a language. Idioms can be professional (only specialists know what they mean), they can be used only by students, children, etc. Slang - something that is not appropriate to the standard form of a language or to formal contexts, and is characteristically more metaphorical and transitory than standard language, is a special part of a language.