I am asking this question in reference to this question.
So as to the general rule, is a question that requires knowledge of legal principles or ideas on topic even though law is not the central part of the question?
If it is not about the law, legal process or legal professions as determined by previous meta discussions and the FAQ/help centre, it isn't on-topic here.
That question in particular is clearly about the meaning of a regular English idiom. That it was used in a legal context doesn't bring it within scope of this site, any more than the use of numbers in a law would make the meaning of a mathematical equation on-topic.