Definition of Frank marriage
In old English law, a species of entailed estates. When tenements are given by one to another, together with a wife, who is a daughter or cousin of the donor, to hold in frank-marriage, the l donees shall have the tenements to them and the heirs of their two bodies begotten, i.e., in special tail. The word "frank-marriage,” ex vi termini, both creates and limits an inheritance, not only supplying words of descent, but also terms of procreation. The donees are liable to no service except fealty, and a reserved rent would be void, until the fourth degree of consanguinity be passed between the issues of the donor and donee, when they were capable by the law of the church of intermarrying. 2 Bl.Comm. 115.
That's the definition of Frank marriage in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.