"of sound mind, mentally sound; free from disorder," 1721, a back-formation from sanity or else from Latin sanus "sound, healthy," in figurative or transferred use, "of sound mind, rational, sane," also, of style, "correct;" of uncertain origin. Perhaps from PIE *seh-no- from *seh- "to tie." That reconstruction "is purely mechanical," according to de Vaan, the meaning might be "which is in place, in order." Or it could be from a different root meaning "to satisfy" as in Latin satis "enough." Used earlier, of the body, with the sense of "healthy" (1620s). Related: Sanely.