Latin word mos comes from Proto-Indo-European *m-et-

Detailed word origin of mos

Dictionary entry	Language	Definition
*m-et-	Proto-Indo-European (ine-pro)
mos	Latin (lat)	(transf.) precept, law, rule. (transf.) quality, nature mode, fashion. Conduct, behavior, character. Humor, self-will, caprice. Manner, custom, way, usage, practice, habit.