1"mens sana in corpore sano": a healthy mind in a healthy body; in later use frequently specifically with allusion to an approach to education in which this principle is emphasized. Also (occasionally) "mens sana in sano corpore".
2Simply: a healthy mind. Usually allusively.
Early 17th century; earliest use found in John Harington (bap. 1560, d. 1612), courtier and author. From classical Latin mēns sana healthy mind from mēns mind + sana, feminine of sanus healthy; the phrase given at sense 1 occurs in Juvenal Satires 10. 356.