Visuddhimagga XXII-55

Agatīti chandadosamohabhayehi akattabbakaraṇassa, kattabbākaraṇassa ca adhivacanaṃ. Tañhi ariyehi agantabbattā agatīti vuccati.

「不应行」──即由于欲、瞋、痴、怖畏、不应作而作，应作而不作的语义相同。因为圣者不应以此而行，故说不应行。

Ñ(XXII,55): Bad ways is a term for doing what ought not to be done and not doing what ought to be done, out of zeal (desire), hate, delusion, and fear. They are called 'bad ways' because they are ways not to be travelled by noble ones.