not even one: no one, nothing, none at all
This is the strongest sweeping "no" in the Greek New Testament. It is built from the words for "and not" and "one," so at heart it says "not even one." That single idea then bends to fit whatever is being denied.
When the thing being denied is a person, it means "no one, nobody" (no one has ever seen God). When it is a thing, it means "nothing" (apart from me you can do nothing). Placed in front of a noun it works like the word "no" in English, sweeping away every instance (no rest, no fault, no other church).
From there it stretches two more steps. It can deliver a verdict that a person or thing amounts to "nothing," that is, counts for nothing (if I have no love, I am nothing; an idol is nothing in the world). And it can work almost like the phrase "not at all," denying that an action helps or matters even slightly (the flesh profits nothing; the heir is no different from a slave).
All of these are the same denial, "not one," aimed at different targets, which is why one word covers so much ground without ever splitting into unrelated meanings.