Consider the following sentence as an example. I used some hair spray. What part of speech is hair? Intuitively, I want to say it's an adjective modifying spray since hair spray is two separate words and not a compound noun. Hair spray however, as paired nouns, is something that we've decided to call a thing suggesting it might be a noun or maybe some other part of speech as part of a noun cluster (if such a thing exists). A dictionary entry for hair does not describe hair as an adjective. I can think of several more examples like this:

money order, pocket comb, tip money.

Are money, pocket and tip adjectives or something else?

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    This is a hairy subject! – Hot Licks Feb 3 '19 at 16:31
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    Relevant:confusing examples of noun modifiers – Cascabel Feb 3 '19 at 16:37
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    Who says hair spray isn’t a compound noun? It absolutely is. – Janus Bahs Jacquet Feb 3 '19 at 16:42
  • Compound words can be open, closed, or hyphenated. Sometimes open compounds gradually become closed over time (e.g., hairspray per ODO; hair spray per M-W). Compare and contrast compound nouns with noun adjuncts. An article on the latter: thoughtco.com/what-is-attributive-noun-1689012. (BTW, I can't edit in 5 mins, so I delete and reenter; there's no help for it, sorry.) – KannE Feb 3 '19 at 17:06
  • I see. I've never known open compound words were a thing. The reason I posed the question the way I did was to remove any potential political bias from someone's answer so I asked something analogous. My real query concerns the term the Democrat Party. (A pejorative created by Republicans originating in the '60s in the USA. The emphasis is placed on the rat syllable.) A critique is that it is grammatically incorrect and is properly the Democratic Party, democratic being the adjectival form of democrat and historically, the name of the party. Calling it a compound noun would make it acceptable. – Aaron Annecharico Feb 3 '19 at 17:25

Compound nouns is when a noun forms a new noun when modified by either an adjective or, as in this case, by a noun. You can be more specific saying that it is a compositional compound noun, as opposed to a non-compositional noun phrase like red herring. You compose the meanings of hair and spray and can deduce the meaning of the resulting noun phrase hair spray. This is in contrast to red herring, which is neither red nor a herring.

It is also an spaced compound. The two nouns are written separate. It is a descriptive endocentric compound, since hair is modifying spray and the meaning ends up being a type of spray.

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