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]