The Collaborative International Dictionary of English v.0.48:

Frank-marriage \Frank"-mar"riage\, n. [Frank free + marriage.]
   (Eng. Law)
   A certain tenure in tail special; an estate of inheritance
   given to a man his wife (the wife being of the blood of the
   donor), and descendible to the heirs of their two bodies
   begotten. [Obs.] --Blackstone.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

FRANK-MARRIAGE, English law. It takes place, according to Blackstone, when
lands are given by one man to another, together with a wife who is daughter
or kinswoman of the donor, to hold in frank-marriage. By this gift, though
nothing but, the word frank-marriage is expressed, the donees shall have the
tenements to them and the heirs of their two bodies begotten that is, they
are tenants in special tail. It is called frank or free marriage, because
the donees are liable to no service but fealty. This is now obsolete, even
in England. 2 Bl. Com. 115.