「まじ」as "will not"

Question

In Shiina Ringo's song 「長く短い祭」 (Nagaku Mijikai Matsuri), she sing the following line :

忘るまじ、我らの夏を

I already knew the word 「まじ」 as in "seriously", but I found two different translations of the sentence on the internet, more or less professional, that translate it as :

Our summer won't be forgotten

and

We're seriously losing our summer

I did not know that it could be used as "will not" or "must not" and would like to know if it was the right translation in this context.

Here is the link for the song : https://youtu.be/n9J4XGey-z8

Answer 1

Auxiliary まじ stands for either negative conjecture ("must not") or negative volition ("will not"). So, 忘るまじ、我らの夏を means either "we won't forget our summer" or "we mustn't forget our summer", but never "we're seriously losing our summer".