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 do 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