Morganatic, already in use in English by 1727 (according to the Oxford English Dictionary), is derived from the medieval Latin morganaticus from the Late Latin phrase matrimonium ad morganaticam and refers to the gift given by the groom to the bride on the morning after the wedding, the morning gift, i.e., dower. The Latin term, applied to a Germanic custom, was adopted from the Old High German term *morgangeba (modern German Morgengabe), corresponding to Early English morgengifu. The literal meaning is explained in a 16th-century passage quoted by Du Cange as, "a marriage by which the wife and the children that may be born are entitled to no share in the husband's possessions beyond the 'morning-gift'".Oxford English Dictionary, 3rd EditionPhilological Society. A New English Dictionary on Historical Principles. Morganatic. Clarendon Press, 1908. p. 663.
