The Collaborative International Dictionary of English v.0.48:

Retainer \Re*tain"er\, n.
   1. One who, or that which, retains.
      [1913 Webster]

   2. One who is retained or kept in service; an attendant; an
      adherent; a hanger-on.
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   3. Hence, a servant, not a domestic, but occasionally
      attending and wearing his master's livery. --Cowell.
      [1913 Webster]

   4. (Law)
      (a) The act of a client by which he engages a lawyer or
          counselor to manage his cause.
      (b) The act of withholding what one has in his hands by
          virtue of some right.
      (c) A fee paid to engage a lawyer or counselor to maintain
          a cause, or to prevent his being employed by the
          opposing party in the case; -- called also retaining
          fee. --Bouvier. --Blackstone.
          [1913 Webster]

   5. The act of keeping dependents, or the state of being in
      dependence. --Bacon.
      [1913 Webster]