What determines whether something is a "noun adjunct" or just a garden-variety adjective? Does it matter in any meaningful way?
Here is my hypothesis, but I can't find any authoritative source to back it up. I'm hoping someone here can weigh in more definitively.
- Classification of noun adjuncts is based subjectively on whether the word is in "common usage" as a noun. "Book" and "chicken" are commonly nouns and would be considered noun adjuncts in "book collector" and "chicken soup"; "yellow", not so much.
- Noun adjuncts may share some common qualities that differentiate them from other adjectives (like not being able to be inflected into superlative forms... one can't be a "book-est collector"). But they are still fundamentally adjectives and may even appear in the dictionary as such if the adjectival usage is common enough.
What I've found so far...
Wikipedia defines a "noun adjunct" as:
an optional noun that modifies another noun; it is a noun functioning as an adjective.
But English words are not decreed to be nouns or adjectives by some higher authority. So it seems strange that one can prescriptively conclude that something "is a noun" in the first place, let alone extrapolate that it "is a noun functioning as an adjective".
The wiki article cites "chicken soup" as an example of a noun adjunct, but at least one dictionary gives a definition for "chicken" as:
adj. (of food) containing, made from, or having the flavor of chicken
So it seems that there are some differing points of view on how to categorize these words.