Which meaning of 'all but' in 'all but unheard of?

Question

What does '‘end of the Eastern Bloc’ suggest? - Test 3, Q21, p 143 reveals another English issue:

The 95 Theses were written in 1517 and by 1521 [Martin] Luther had developed these ideas and burned the Book of Canon Law and the Papal Bull Exsurge Dominie. Four years in the C20 would be considered relatively quick for the weakening of an established order. The end of the Eastern Bloc did not happen overnight. In the C16 such an occurrence was all but unheard of. However, on closer analysis, arguments against this approach can be forwarded.

How do you determine which definition of all but applies? Please explain the steps or thought processes; I’d like to try to resolve this myself in the future? The Oxford online dictionary lists two meanings:

1 Very nearly: the subject was all but forgotten

2 All except: we have support from all but one of the networks

What are the similarities and differences between these two meanings? From that other question, I think a different meaning applies; it seems like all but just intensifies how 'unheard of' it was:

In the C16 such an occurrence was ... unheard of.

Source: p 142, Mastering the National Admissions Test for Law, Mark Shepherd

Answer 1

The first definition within the link applies. The phrase "all but" means "almost completely". It DOES NOT intensify how unheard of it was, in fact, it does the opposite. It means that some people had heard of it, but most had not.

The differences between the two definitions are as follows, 1st:

Almost all

2nd:

Apart from

The link does a good job at showing the differences, but I hope my examples are more understandable.

Answer 2

Both of the Oxford dictionary possibilities ¹, quoted in the question, result in one meaning for the phrase in the given context: "this is rare". You should affirm this by looking at the context². It is clear, in the preceding construction of the question, that the author is saying such an occurrence, in the 16th century, is rare. Then, replacing "all but" with the suggested alternatives:

1. In the 16th century such an occurrence was very nearly unheard of.
2. In the 16th century such an occurrence was all except unheard of.

While the latter is perhaps less comfortably understood, it too supports and validates the preceding argument that the occurrence is rare. The 2nd construction might be understood otherwise, IF it were standalone, but it is not. The context is important, especially for such constructions.

¹

1 Very nearly: the subject was all but forgotten

2 All except: we have support from all but one of the networks

² For readers newly coming to this question, replacing C16 and C20 with 16th century and 20th century, respectively, should clarify the question. The use of C16 and C20 is generally obtuse, and is likely understandable only because of previous use in the context of the instructional environment.