The phrase "unsound mind" means that someone's thinking or mental state is not healthy or normal, and they may have difficulty making rational decisions or understanding reality. Full definition
So Wolfgang Whatever woke up one bright summer morning and in a fit of unsound mind spontaneously decided to resign because somebody on a blog had said a nasty thing about a paper published in his journal?? (judithcurry.com)
Unsound minds do not help societies, rather work like a cancer against it. (religion.blogs.cnn.com)
He secured the passage of the Lunacy Act of 1845, which for the first time treated the insane as a «person of unsound mind», rather than as a social outcast. (religion-online.org)