unsound mind

insane, mentally unstable, lunatic

UNSOUND MIND, MEMORY - These words have been adopted in several statutes, and sometimes indiscriminately used to signify, not only lunacy, which is periodical madness, but also a permanent adventitious insanity as distinguished from idiocy.

The term unsound mind seems to have been used in those statutes in the same sense as insane; but they have been said to import that the party was in some such state as was contradistinguished from idiocy and from lunacy, and yet such is made him a proper subject of a commission to inquire of idiocy and lunacy.
   

This entry contains material from Bouvier's Legal Dictionary, a work published in the 1850's.