What does 'must make do' means?

Question

Thus there is no near-contemporary source of Anglo-Saxon origin. The reason is obvious enough: the Germanic peoples were illiterate during their first two centuries in Britain. So their early fortunes can only be glimpsed through the hostile eyes of Britons, through the ill-informed eyes of foreigners, and by means of their own half-remembered traditions. Until the late sixth century, informed guesswork must make do for history.

In the last sentence, I don't understand what does the phrase 'must make do' means. Is it an unformal expression?

Answer 1

Must make do means that you have to do with what you have. In this piece of writing, it means that we have to rely on informed guesswork since that's all we have. We'll have to just use what we have. It is considered informal.

Answer 2

Yes, make do is a set phrase. According to the Cambridge dictionary, it means

to manage to live without things that you would like to have or with things of a worse quality than you would like

Note that, in normal usage, the people who have to live without something are the subject, and the things that they have to live without are attached using the preposition with.

The final sentence in your question is somewhat unusual because the thing that you have to live without is the subject: it would be closer to normal usage written like this:

For history until the late sixth century, we must make do with informed guesswork.