The Collaborative International Dictionary of English v.0.48:

Usurp \U*surp"\, v. t. [imp. & p. p. Usurped; p. pr. & vb. n.
   Usurping.] [L. usurpare, usurpatum, to make use of, enjoy,
   get possession of, usurp; the first part of usurpare is akin
   to usus use (see Use, n.): cf. F. usurper.]
   To seize, and hold in possession, by force, or without right;
   as, to usurp a throne; to usurp the prerogatives of the
   crown; to usurp power; to usurp the right of a patron is to
   oust or dispossess him.
   [1913 Webster]

         Alack, thou dost usurp authority.        --Shak.
   [1913 Webster]

         Another revolution, to get rid of this illegitimate and
         usurped government, would of course be perfectly
         justifiable.                             --Burke.
   [1913 Webster]

   Note: Usurp is applied to seizure and use of office,
         functions, powers, rights, etc.; it is not applied to
         common dispossession of private property.
         [1913 Webster]

   Syn: To arrogate; assume; appropriate.
        [1913 Webster]