How should I understand the phrase "это не суть важно"?

Question

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

1. whether the phrase "это не суть важно" is grammatically correct,

2. if the phrase is correct, how the grammar works in this phrase,

3. 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,"

4. the formal meaning of the phrase, and

5. 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.

Answer 1

My attempt:

whether the phrase "это не суть важно" is grammatically correct,

No. But yes, it's a relatively widespread phrase. I believe I've heard people saying it.

if the phrase is incorrect, why people use it that often, strongly preferring it to "это не есть важно,"

They don't (usually) realise that it's grammatically incorrect and/or don't care. They just like using this phrase and use it (knowing that they will be understood). See more on this below*.

what people actually want to say with the phrase, if different from (4)?

They mean "это не важно", "это не очень важно", "не в этом суть"...

*- Basically, this is "не в этом суть" + "это не важно" (upd1 probably also influenced by "не столь важно", which sounds similar) glued together in the sayer's mind. I struggle to give a proper term for such a language phenomenon off the top of my head but the gut feeling tells me that such a term exists, as well as other examples of the phenomenon. Intuitively, I would call it a variant of tautology.

"Это не есть важно" is not used because "есть" is strongly felt as being redundant here. But some individuals could say that. More often you might hear the phrase "это не есть хорошо". These are the examples of … well, some people just want their speech to be "with a twist", they just like "breaking the rules" slightly. Thinking, probably, that this makes them more interesting or sound a bit more clever. When such phrases are overused I personally find this rather annoying and silly.

Answer 2

1. It isn't. Like @turn_ said, суть an old and plural present-tense form of the verb быть, which strictly speaking has no business being used in the singular.

2. It doesn't work. But in your authentically old postscripted example, суть correctly agrees with the plural они.

3. Gaps in the understanding of one's own older language happen all the time. As do occasions when people consciously want to sound archaic. This results in "broken" archaic language. In English, you get ye olde, which isn't a form that ever actually existed; it came from a relatively modern misreading of þe olde, þ being just a fancy old single-letter th. Or, to take a closer example, lots of people would say I thinketh to imitate Shakespearean-era English, which is just as broken as "I thinks".

4. Это не суть важно can be translated, with a minor adaptation, as It art not important. Basically, the speaker mistakenly thought art (2nd person sg. pres.) was just a way of saying is with some added archaic gravitas.

5. The intended effect is something like "it matters not" or "it is of no importance" — i.e. "it doesn't matter" said with a manneristic self-importance.

Answer 3

In Proto-Indo-European the 3rd person singular of "is" was e̯esti and 3rd person plural was e̯sont.

The first became есть in Russian and the second became суть. Thus, using суть for singular is wrong even in historical context.

The modern Russian noun суть came from the PIE noun e̯sntia̯ meaning "being" or "essence".

Answer 4

In old russian the verb "быть" had a форм "суть" for plural and 3rd person: "Я есть" ("Аз есмь" for even older), "Они суть", "Он суть", "Оно суть". Especialy becase it can be easely misundestood with "to eat" verb: "Оно есть" - "Оно ест". ("Это не ест важно" - "It doesn't eat lordly" is a strange, but valid phrase )