Русско-англо-русский юридический словарь

sound mind and memory – n. having an understanding of one's actions and reasonable knowledge of one's family, possessions and surroundings. This is a phrase often included in the introductory paragraph of a will in which the testator (writer of the will) declares that he/she is "of sound mind and memory." (The People's Law Dictionary by Gerald and Kathleen Hill)

He concluded that Serabjit-Singh was of “sound mind and memory” and was not under “any undue influence” when she signed her last will. (The Buffalo News)

The expression “of sound mind and disposing memory” was originally a term of art in law in the context of making a Last Will and Testament. Originally, it meant a good mental ability (sound mind) with a capable memory (disposing memory) for the legal purposes of making a Will. [...] The ultimate reason for using “disposing” was the idea that the person’s memory had to be capable of arranging or placing things for that particular legal purpose — from the Old French disposer, which was from Latin disponere (‘arrange’) (Quora)