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
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.
- blowsy coarse and flushed. From blowze, a bloated slattern
- bousy drunk. From bouse, liquor or a drinking bout
- busy No known related noun, from the Old English adjective besig
- 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.
- cosy "derivation unknown"
- cutesy No noun, from cute, an adjective
- drowsy From drowse, a fit of sleepiness, itself from the verb of the same spelling
- 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.
- queasy "Of obscure history"
- teensy Likely from the obsolete tine, very small or something very small
- weensy No related noun. Likely from wee and a rhyme with teensy