Not sure the title is very clear so I’ll give the example I ran across (bolding mine).
“The board of directors deep-sixed the proposal without even reading it.” This phrase is derived from the noun “deep six,” meaning burial at sea…" (source)
Seems…odd.
I can kind of see it but then, well, color me confused.
Is that correct? Shouldn’t a “noun” such as that properly be hyphenated or a compound word to denote it is “one” thing?
Try this. Noun phrase.
Yes.
I am not a grammar expert, but “deep six” is an idiom. (You can’t derive its meaning from simply looking up the adjective “deep” and then looking up the adjective “six” to figure out what it means.) Even so, it is still two separate words. Normally you add a hyphen to use a noun phrase as an adjective, especially when needed to avoid ambiguity, such as in the phrase “other idiomatic two-word nouns.” But it’s not necessary to have a hyphen just to make clear that a phrase is a noun.
Other idiomatic two-word nouns:
Give him the bum’s rush.
I was on cloud nine.
In my elementary school we were taught about closed and open compounds. A closed compound is one word, but an open compound may be two words that constitute one idea–the example I remember is “hot dog” which is not a warm canine so the words together take on a different meaning from each individual word.
Ever been 86ed?
Some nouns actually started as two distinct words that gradually became one: think base ball >> base-ball >> baseball.
I’d be willing to make a bet that “home page” becomes “homepage” sooner rather than later, and will eventually be used as a verb. (“Kewl! She homepaged me on Facebook.”)
Isn’t “deep sixed” being used as a verb in that sentence?
I had rather be 69ed than 86ed! 
There is one beautiful if obsolete word that is always written as two discrete words which illustrates the point: pine apple. The word is synonymous for pine cone, “apple” being used here in an obsolescent meaning of “tree fruit” (as in what Adam and Eve ate, for example) rather than the specific couple of species it has become restricted to. And the Dole company’s product was termed “pineapple” as a single word because its spiky external integument reminded sailors of the pine apples growing on evergreen trees back home. For obvious avoidance-of-confusion reasons the term for seed-bearing body of the pine tree remained a two-word noun (and was rapidly replaced by the aurally distinct “pine cone”).
Your source is wrong about the origin of “deep six”. In fact this originates from a sounding line, used to measure water depth from a ship.
The line had marks for various fathoms; when the line was cast and the depth came out near one of the marks, the cry was “By the mark, [xx]” where xx was the depth in fathoms. Fathoms between the marks were indicated with “By the deep, [xx]”.
It would be strange indeed to bury someone in only 6 fathoms (36 feet) of water.
The Wiki article seems to be addressing the noun phrase as a part of speech with regard to parsing a sentence, rather than word usage. The example “red ball” is a modified noun and functions as a noun in a sentence but is not the same type of thing that the OP is getting at. In fact, neither “deep” nor “six” is a noun so it’s not really a noun phrase grammatically even though it’s a phrase that acts as a noun.
Yes, but the OP is asking, “Why is there no hyphen when it is being used as a noun, such as in the referenced source?”
Yes, it is.
Well, yeah, but 1) it took me about three seconds after reading the OP to come up with a cite demolishing the notion that nouns have to be one word; 2) nobody in over 24 hours has found a term for “a phrase that acts as a noun;” and 3) I never said it was a noun phrase in the first place.
Other than that… 
I have been mulling your cite and am not sure it addresses the OP.
For instance, your cite indicates “The blonde girl shouts” that “the blonde girl” constitutes a “noun phrase”. While together they act like that clearly “blonde” is an adjective modifying the noun “girl”.
In the OP none of the words is a noun but in that configuration they are saying it is a noun.
I think what you cited as a response does not address the OP.
Of course I am not an expert in this but seems that way to me.
What about “grass hut”, “stone building”, “metal plate”, “rubber tire”, “wool sweater”. Each phrase is a noun and each word in each phrase is itself a noun.
I am tempted to say that the first word in each of these phrases acts as an adjective but that’s where I call in the experts.
In all cases you list, in that usage, the first word is an adjective. “Grass” describes what kind of “hut” it is. And so on.
ETA: Drat…beaten to the punch. Hamsters are slow today.
Which dictionary says that the word “grass” is an adjective?
Here’s the trick: “Noun phrase” covers a number of different constructions, which have in common a noun at the heart of them:
[ul][li]“The duck” – simple noun with article[/li][li]The white duck" – noun modified by adjective[/li][li]“The farmer’s duck” – noun modified by possessive noun used adjectivally[/li][li]“The Peking duck” – noun modified by another noun functioning adjectivally[/li][li]“The cold duck” – presuming we’re speaking of an alcoholic beverage, not a frigid waterfowl, what we have here is a two-word unit carrying a meaning not discernible from its constituent parts. [/ul][/li]
Obviously the last case is something distinct from the previous four cases. It’s a noun, an identifying marker referencing some object (or concept, as in “homophobia”) which is comprised of more than one unit but which does not derive its meaning from the combination of its constituents’ meanings, as cases #2, #3, and #4 do.
And that is the point at which “word”, in the common-usage/typographical meaning “sequence of letters with spaces fore and aft, unless punctuation precedes the aft space,” diverges from the analytical-grammar meaning of “unbound morpheme.” In such cases the meaning of the supposed phrase is not discernible from the constituent elements.
It is an adjective as this explains:
The solitary word “grass” is still a noun, though. It only acts as an adjective when you put it into the noun phrase “grass hut”.