Mimsy were the Borogoves - why is "mimsy" an adjective?

I'm reading Lewis Carroll's "Through the Looking-Glass" and I've found a famous poem Jabberwocky:

Twas bryllyg, and the slythy toves
Did gyre and gymble in the wabe:
All mimsy were the borogoves;
And the mome raths outgrabe.

I've read some articles about that poem. All of them (for instance, this one) say that mimsy is an adjective, but they don't say why. I can't make head nor tail of the syntax of this sentence.

For example, a sentence with similar structure might look like this:

All gray were the wolves

But I guess that it is an incorrect form, and the correct form is:

All the gray ones were the wolves

So, is the form of using an adjective in the example above really correct? And why it can't be a noun (i.e. like in all wolves were the animals)? I really can't understand it.

At the suggestion of the kind commenter, let me discuss the syntax of

All mimsy were the borogoves

There are two possibilities. The first is that mimsy is a noun, in which case we have the structure

Subject Copulative-Verb Predicate-Complement

where

Subject = Noun Phrase All mimsy
Copulative Verb = were
Predicate Complement = Noun Phrase the borogoves

This pattern would obtain for recognizable nouns thusly:

All priests were the granters of indulgences.

The second possibility is that mimsy is an adjective. Ordinarily we would expect the same word order when the complement is an adjective, i.e.,

The borogoves were all mimsy

but it's possible to invert the order and put the adjectival complement all mimsy first, i.e., to have the order

Predicate-Complement Copulative Verb Subject

In recognizable words:

All rosy were the cheeks of the children.

This is a recognizably poetic style. Consider the lines from Walt Whitman's Leaves of Grass:

Brave, brave were the soldiers (high named to-day) who lived through the fight;
But the bravest press'd to the front and fell, unnamed, unknown.

In ordinary prose, we'd say "the soldiers were brave".

At the prompt of a second kind commenter, let me note that the modifier all doesn't help us figure out whether to choose noun or adjective for mimsy. (Perhaps this part of the sly whimsy of the Deacon Dodgson.) If we're thinking noun, then all is an acceptable as a universal determinative, as in

All soldiers are brave.

If we lean toward adjective, then all is acceptable as an adverbial modifier, as in

We are all excited.

(Note the ambiguity. In an intensifying role -- We are all very excited -- the Ngram viewer finds few examples in the 19th century, and it seems to be an American colloquialism. The universal role -- We are all of us excited -- is common and earlier.)

To decide, consider the following common^* English adjectives, most^‡ of which are formed by adding the suffix -y to the related noun ending in s/se to get an adjectival form: ease->easy, nose->nosy, etc.

antsy artsy backwoodsy bitsy blousy blowsy¹ bluesy booksy bossy bousy² brassy busy³ cheesy choosy citrusy classy clumsy⁴ cosy⁵ creasy cutesy⁶ ditsy dressy drowsy⁷ easy flimsy⁸ flossy folksy fussy gassy glassy glossy gneissy goosy grassy greasy gutsy hissy kissy lossy lousy messy minstrelsy mossy mousy mussy newsy noisy nosy outdoorsy primrosy prissy queasy⁹ rosy sassy sudsy teensy¹⁰ tipsy tricksy weensy¹¹ woodsy wussy

Now compare this to a list of common^** English nouns ending in -sy:

apostasy autopsy biopsy catalepsy controversy courtesy curtsy daisy dropsy dyspepsy ecstasy embassy epilepsy fantasy footsy geodesy gypsy heresy hypocrisy hypostasy idiosyncrasy isostasy jealousy leprosy narcolepsy necropsy palsy pansy pleurisy poesy pussy speakeasy whimsy

The countable nouns require a plural form to go with the determinative all and the plural verb were:

All autopsies were performed by the coroner.
All fantasies were repressed by the patients.

and so on. The noncountable nouns in the singular can take the preceding all, but they require a singular verb:

All fantasy is meaningless.
All courtesy is hypocritical.

Given the distribution favoring adjectives of words ending in -sy, the lexical likelihood of finding a related noun, the syntactic constraints on number, and the fact that this is a poem, where the subject/adjectival-complement order may be inverted, a fluent English speaker will consider that mimsy is an adjective meaning having the qualities of a mim, some noun unknown.

*I obtained my list here. Since I edited the results, and I'm going to argue by weight of vocabulary, I need to reveal my method. I avoided double counting by excluding some words formed by prefixing (e.g., aacatalepsy, discourtesy, nondrowsy, overbusy, rebiopsy, ultraglossy, uneasy).

**I eliminated technical (mostly medical) terms (biorhexistasy, cholecystolithotripsy, cholelithotripsy, dyschromatopsy, glacioisostasy, hemiachromatopsy, hemichromatopsy, hypnolepsy, lithotripsy, nephrolithotripsy).

‡ Those not related to common nouns are noted in the numbered notes, the content of which is from the OED. Those which have no known candidates for related nouns are in bold italic.

1. blowsy coarse and flushed. From blowze, a bloated slattern
2. bousy drunk. From bouse, liquor or a drinking bout
3. busy No known related noun, from the Old English adjective besig
4. clumsy probably from the now-obsolete clumse, eventually from a Scandinavian word meaning to benumb with cold. It had a number of pejorative uses as an adjective and an attributive noun.
5. cosy "derivation unknown"
6. cutesy No noun, from cute, an adjective
7. drowsy From drowse, a fit of sleepiness, itself from the verb of the same spelling
8. flimsy Derivation uncertain. The claim of film seems like folk etymology, especially given the adjective filmy. But the word flim is preserved as a fossil in flim-flam, and it’s possibly related to an Old Norse word flim meaning a mockery or lampoon.
9. queasy "Of obscure history"
10. teensy Likely from the obsolete tine, very small or something very small
11. weensy No related noun. Likely from wee and a rhyme with teensy

Later in the book, Humpty Dumpty gives Alice an explanation of the odd words in the poem and he defines 'mimsy' as 'miserable and flimsy'. In other words, we know it is an adjective because Lewis Carroll intended it to be one.

Fortunately, Carroll's own definition assigns the part of speech to 'mimsy'. Otherwise, the word might now be taken as an early and unprecedented appearance of the British regional 'mimsy', also an adjective, but with a somewhat different meaning, although latterly sometimes influenced by Carroll's coinage:

Prim; careful; affected; feeble, weak, lightweight.

["mimsy, adj.2 (and adv.)". OED Online. September 2016. Oxford University Press. http://www.oed.com/view/Entry/255344 (accessed November 08, 2016).]

This 'mimsy' is attested first in 1880; the etymology is given:

Apparently < MIM adj. + -sy (in CLUMSY adj., FLIMSY adj., TIPSY adj., etc.). Compare MIMSY adj.¹, by which this word appears sometimes to be influenced.

(op. cit.)

The first part of the compound, 'mim', has a much earlier provenance, first appearing around 1586, and attested through 1991. It is also an adjective (and adverb), used regionally in Scottish and British English:

Reserved or restrained in manner or behaviour, esp. in a contrived or priggish way; affectedly modest, demure; primly silent, quiet; affectedly moderate or abstemious in diet (rare). Also (occas.) of a person's appearance.

["mim, adj. and adv.". OED Online. September 2016. Oxford University Press. http://www.oed.com/view/Entry/118631 (accessed November 08, 2016).]

However, Carroll himself was kind enough to differentiate between his 'mimsy' and the British regional 'mimsy'. As Humpty Dumpty explained to Alice, 'mimsy' is a portmanteau of 'flimsy' and 'miserable' (Through the Looking Glass, 1871):

Exactly so. Well, then, "mimsy" is "flimsy and miserable" (there's another portmanteau for you).

The portmanteau clearly amounts to something akin to 'unhappy', but if there was any doubt, that meaning is made explicit in Carroll's gloss of the first verse of "Jabberwocky", first published in Carroll's own literary magazine, Mischmasch, in 1855.

As mentioned by Carroll's nephew, Stuart Collingwood, who provides a facsimile of part of the gloss in an article in The Strand Magazine ("Before 'Alice' — The Boyhood of Lewis Carroll", 1899, p. 616), Carroll himself defined the eleven words in the verse that were not "pure, honest English":

(op. cit.)

Collingwood's account gives the rest of Carroll's gloss, and also supplies a complete 'translation' of the first sentence (continuing from the facsimile):

The evidence from the 1855 Mischmasch, along with the evidence given by Humpty Dumpty in the 1871 Through the Looking Glass, leaves no doubt that 'mimsy' is an adjective. Without that evidence, 'mimsy' could well be a noun, verb or even a subdued interjection in poetic use.