Just encountered in some Japanese text. I understand it's conjugated in the base negative form of the verb but I'm baffled as to what the じ means.
It's a Classical Japanese particle, nowadays only used as frequent as "methinks".
It means exactly what must not (as in 'I suppose the pie must not be ready yet') does in English, and attaches to Classical 未然形 of a verb or adjective (that is, the form which the negative -ず and -ぬ appends to).
See: the entry of じ in a Japanese-Japanese dictionary