c.1300, "belonging to all, general," from O.Fr. comun "common, general, free, open, public" (9c., Mod.Fr. commun), from L. communis "in common, public, general, not pretentious, shared by all or many," from PIE *ko-moin-i- "held in common," compound adjective formed from *ko- "together" + *moi-n-, suffixed form of base *mei- "change, exchange", hence literally "shared by all."
late 14c., from O.Fr. comunion "community, communion" (12c.), from L. communionem (nom. communio) "fellowship, mutual participation, a sharing," used in L.L. ecclesiastical language for "participation in the sacrament," from communis. Used by Augustine, in belief that the word was derived from com- "with, together" + unus "oneness, union."