M'Naghten rule

Noun

McNaughton rules pl (plural only)

1. (plural, plural-only, criminal law) A test of criminal insanity by which "it must be clearly proved that, at the time of committing the act, the party accused was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing, or, if he did know it, that he did not know he was doing what was wrong".