I've never come across this particular example either, but it is a general truth in English that words do not have fixed grammatical functionality as they do in most other languages. For example, the world 'table' in French IS and MUST BE a noun, but in English we already commonly use it as either a noun or verb and we may use it pretty much any way we like and be understood. In the expression "table tennis" for example, it's being used as an adjective. English so easily enables this function-switching that we are immediately understood when we invent new uses, such as "Oh, that's very Lady Di" (using a proper noun as an adjective).