"Sometimes the opinions are undutiful."
Translation:Interdum sententiae sunt impiae.
It's the opinion expressed by someone who is undutyful, as a wise idea or a wise opinion, would be expressed by someone who is wise, a respectful opinion, by someone who is respectful, or express respectfully, or respectful concepts, an impious idea, etc...
For ideas/opinions/conversations, etc..., the adjective describing morally someone can apply, even if they don't apply usually for inert things.
Do you have a dictionary page for this meaning in British English. I'm still learning English, I understand that impiety could be used figuratively, in an informal way, but, for me, "impiety" is still linked to a religious topic.