c. 1600, Latin, literally "a sound mind in a sound body," a line found in Juvenal, "Satires," x.356.

Mens sana in corpore sano is a contradiction in terms, the fantasy of a Mr. Have-your-cake-and-eat-it. No sane man can afford to dispense with debilitating pleasures; no ascetic can be considered reliably sane. Hitler was the archetype of the abstemious man. When the other krauts saw him drink water in the Beer Hall they should have known he was not to be trusted. [A.J. Liebling, "Between Meals," 1962]

early 15c., permitten, transitive, "allow (something) to be done, suffer or allow to be," from Old French permetre and directly from Latin permittere "let pass, let go, let loose; give up, hand over; let, allow, grant, permit," from per "through" (from PIE root *per- (1) "forward," hence "through") + mittere "let go, send" (see mission). Meaning "grant liberty or leave" is from 16c. Related: Permitted; permitting.

c. 1600, "allowing to pass through," from Medieval Latin *permissivus, from Latin permiss-, past-participle stem of permittere "to let go, let pass, let loose" (see permit (v.)). In sense of "tolerant, liberal" it is attested by 1946; by 1966 it had definite overtones of sexual freedom. Earlier it meant "permitted, allowed" (mid-15c.). Related: Permissively; permissiveness.