The Collaborative International Dictionary of English v.0.48:

Ouster \Oust"er\, n. [Prob. fr. the OF. infin. oster, used
   substantively. See Oust.]
   A putting out of possession; dispossession; disseizin; -- of
   a person.
   [1913 Webster]

         Ouster of the freehold is effected by abatement,
         intrusion, disseizin, discontinuance, or deforcement.
                                                  --Blackstone.
   [1913 Webster]

   2. Expulsion; ejection; as, his misbehavior caused his ouster
      from the party; -- of a person, from a place or group.
      [PJC]

   Ouster le main. [Ouster + F. la main the hand, L. manus.]
      (Law) A delivery of lands out of the hands of a guardian,
      or out of the king's hands, or a judgement given for that
      purpose. --Blackstone.
      [1913 Webster]

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

OUSTER LE MAIN. In law-French, this signifies, to take out of the hand. In
the old English law it signified a livery of lands out of the hands of the
lord, after the tenant came of age. If the lord refused to deliver such
lands, the tenant was entitled to a writ to recover the same from the lord;
this recovery out of the hands of the lord was called ouster le main.