means The state of having a sound, rational mind — being mentally healthy and able to think and judge clearly.

from From Latin sanitas, 'health' or 'soundness,' built on sanus, 'healthy, whole.' The same sanus underlies 'sane,' 'sanitary,' and 'sanitation' — so cleaning your kitchen and keeping your wits share a Roman root. The phrase mens sana in corpore sano, 'a sound mind in a sound body,' carries the whole idea in miniature. English borrowed 'sanity' to mean bodily health first; the specifically mental sense came later as the word narrowed toward the mind.