The full phrase is:
There was not one in all that assemblage but knew it.
This is from a book first published in 1906
My instinct is that it means “everyone in the assemblage knew”.
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It's syntactically similar to It never rains but it pours (essentially a "frozen form", where the construction is rarely extended to new contexts today).
Unquestionably the "meaning" of but here is except, but even then it's not easy to explain exactly how the syntax works. I think it might help to note that "but" in such contexts is sometimes called a "negative relative pronoun" - which means we can conceptualise it as equivalent to...
there was not one in all that assemblage but [that one] knew it
...where the highlighted element there effectively means any one of all those people [chosen at random].
Semantically, what we have is a pair of negated elements (none of them didn't know it), which would more naturally be expressed by "cancelling out" both the negations (all of them did know it).