expression, noun-modifying (irregular)
Fixed prenominal expression meaning 'against the path (of righteousness)'. Used to describe actions, relationships, or desires that are morally wrong or socially unacceptable. Often appears in set phrases like 道ならぬ恋 (forbidden love).
He fell into a forbidden love.
You must not do immoral deeds.
From 道 (path, way, morality) + ならぬ (classical negative of なる, 'to be'), literally 'not being the way'. The expression is a fossilized classical form meaning 'not in accordance with the proper path'.