The Collaborative International Dictionary of English v.0.48:

Arrogation \Ar`ro*ga"tion\, n. [L. arrogatio, fr. arrogare. Cf.
   Adrogation.]
   1. The act of arrogating, or making exorbitant claims; the
      act of taking more than one is justly entitled to. --Hall.
      [1913 Webster]

   2. (Civ. Law) Adoption of a person of full age.
      [1913 Webster]

WordNet (r) 3.0 (2006):

arrogation
    n 1: seizure by the government [syn: confiscation,
         arrogation]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "arrogation":
   accession, accounting for, adoption, anointing, anointment,
   answerability, application, appointment, appropriation, ascription,
   assignation, assignment, assumption, attachment, attribution,
   authorization, blame, charge, colonization, connection with,
   conquest, consecration, coronation, credit, delegation, deputation,
   derivation from, election, empowerment, encroachment, enslavement,
   etiology, honor, imputation, indent, infringement, invasion,
   legitimate succession, occupation, palaetiology, placement,
   playing God, preemption, preoccupation, prepossession,
   reference to, requisition, responsibility, saddling, seizure,
   subjugation, succession, takeover, taking over, trespass,
   trespassing, usurpation



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

ARROGATION, civil law. Signifies nearly the same as adoption; the only
difference between them is this, that adoption was of a person under full
age but as arrogation required the person arrogated, sui juris, no one could
be arrogated till he was of full age. Dig. 1, 7, 5; Inst. 1, 11, 3 1 Brown's
Civ. Law, 119.