late 14c., "state of being a concubine; act or practice of cohabiting in intimacy without legal marriage," from Old French concubinage, from concubin, from Latin concubina (see concubine). In ancient Roman law, "a permanent cohabitation between persons to whose marriage there were no legal obstacles."
It was distinguished from marriage proper (matrimonium) by the absence of "marital affection"--that is, the intention of founding a family. As no forms were prescribed in the later times either for legal marriage or concubinage, the question whether the parties intended to enter into the former or into the latter relation was often one of fact to be determined from the surrounding circumstances, and especially with reference to a greater or less difference of rank between them. [Century Dictionary]