Non Compos Mentis Definition: Latin: Not of sound mind. Related Terms: Lunatic, Mental Disorder, Mentally Ill, Mentally Retarded, Alieni juris, Competency In law, a person with an unsound mind.Formerly, synonymous with lunatic and, more recently, insanity.Ballentine's 1969 law dictionary:"Non compos mentis: mentally incompetent. Having a mental condition approximating total and positive incompetency. Destitute of memory and understanding.... A person of unsound mind."In Dexter v Hall, Justice Strong of the United States Supreme Court used these words:"[A] lunatic, or a person non compos mentis, has nothing which the law recognizes as a mind, and it would seem, therefore, upon principle, that he cannot make a contract which may have any efficacy as such."REFERENCES:Dexter v. Hall, 82 US 9 (United States Supreme Court, Justice Strong, 1872)Van Deusen v Sweet, 51 NY 378 Categories & Topics: Civil Law Dictionary Contract Law Dictionary Dictionary of Latin Law Terms Medical-Legal Dictionary Tort and Personal Injury Law Dictionary Find you are constantly looking up definitions? Try our search provider (works in most modern browsers) If you find an error or omission in Duhaime's Legal Dictionary, or if you have legal term suggestion, we'd love to hear from you!