I saw the phrase "это не суть важно" in the Internet and then found that this phrase is often used in modern Russian.
I initially supposed that "суть" is an adverb synonymous to "очень", so I initially interpreted the phrase as "это не очень важно." This made perfect sense in the context.
But I got extremely puzzled when I discovered by looking in Wiktionary that суть is not an adverb (Link). Wiktionary says that "суть" can be only a noun or a verb. As a noun, "суть" means "essence." As a verb, "суть" is the archaic third-person plural present tense of "быть." Neither of these meanings seems to fit: As a noun this word simply does not fit into the overall structure of the sentence, and as a verb this word does not fit with "это," because "это" is a singular, not plural, pronoun. For a singular pronoun like "это," the correct form of "быть" is "есть" (third person singular present tense) - "это не есть важно." But Google shows that people choose "суть" in this phrase much more often than "есть." I am so much puzzled by this. I never saw phrases with "это" followed by a plural form of the verb. People say "это будет/влияет/соответствует" and never say "это будут/влияют/соответствуют."
And I am also puzzled as to what people actually want to say with this phrase - "это не важно" ("this is not important"), "это по сути не важно" ("this is essentially not important"), "это не очень важно "("this is not very important"), or something else. I am curious precisely how the meaning of "это не важно" is changed by adding "суть" between "не" and "важно."
My question is this: Could you put all pieces of the puzzle together, telling me
whether the phrase "это не суть важно" is grammatically correct,
if the phrase is correct, how the grammar works in this phrase,
if the phrase is incorrect, how Russian ended up having this idiom, given that idioms are almost never a result of a slip of the tongue and that this idiom is strongly preferred to the correct phrase "это не есть важно,"
the formal meaning of the phrase, and
what people actually want to say with the phrase, if different from (4)?
I humbly hope that for each item I will get answers different from the phrase in question itself.
P.S. Look at this sentence from an old Russian text:
Это близко къ истинѣ , покрайней мѣрѣ , справедливо то , что они не суть понятія общія , отвлеченныя ...
Maybe this kind of "не суть" (meaning "are not") is the key to the idiom "это не суть важно," but the problem is that this kind of "не суть" requires a plural noun/pronoun, for example, "они не суть важны."
P.P.S.: I just found that the expression "не суть важно" is frequently used even in books, even in books of 1800s. For example, Belinsky wrote: "Но быть такъ, это все еще не суть важно..." And Saltykov-Schedrin wrote: "Мы оба требуемъ отъ массъ подчиненія, а во имя чего мы этого требуемъ— во имя ли принциповъ „порядка“, или во имя „жизни духа“ — право, это еще не суть важно." It is hard to accept that these classic Russian writers were uneducated and expressed their thoughts in a way not conforming the grammar norms of the Russian language at that time. You can see numerous examples of use of "не суть важно" in books by following this link.