What is the X in X-mas?
early 15c., "healthy condition," from Middle French sanité "health," from Latin sanitatem (nominative sanitas) "health, sanity," from sanus "healthy; sane" (see sane). Meaning "soundness of mind" first attested c.1600.
sanity san·i·ty (sān'ĭ-tē)
The quality or condition of being sane; soundness of mind.
Soundness of judgment or reason.