The Collaborative International Dictionary of English v.0.48:

Subinfeudation \Sub*in`feu*da"tion\, n. (Law)
   (a) The granting of lands by inferior lords to their
       dependents, to be held by themselves by feudal tenure.
       --Craig.
   (b) Subordinate tenancy; undertenancy.
       [1913 Webster]

             The widow is immediate tenant to the heir, by a
             kind of subinfeudation, or undertenancy.
                                                  --Blackstone.
       [1913 Webster]

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

SUBINFEUDATION, estates, English law. The act of an inferior lord by which
he carved out a part of an estate which he held of a superior, and granted
it to an inferior tenant to be held of himself.
     2. It was an indirect mode of transferring the fief, and resorted to as
an artifice to elude the feudal restraint upon alienation: this was
forbidden by the statute of Quia Emptores, 18 Ed. I; 2 Bl. Com. 91; 3 Kent,
Com. 406.