The Collaborative International Dictionary of English v.0.48:

Arrogant \Ar"ro*gant\, a. [F. arrogant, L. arrogans, p. pr. of
   arrogare. See Arrogate.]
   1. Making, or having the disposition to make, exorbitant
      claims of rank or estimation; giving one's self an undue
      degree of importance; assuming; haughty; -- applied to
      persons.
      [1913 Webster]

            Arrogant Winchester, that haughty prelate. --Shak.
      [1913 Webster]

   2. Containing arrogance; marked with arrogance; proceeding
      from undue claims or self-importance; -- applied to
      things; as, arrogant pretensions or behavior.
      [1913 Webster]

   Syn: Magisterial; lordly; proud; assuming; overbearing;
        presumptuous; haughty. See Magisterial.
        [1913 Webster]

WordNet (r) 3.0 (2006):

arrogant
    adj 1: having or showing feelings of unwarranted importance out
           of overbearing pride; "an arrogant official"; "arrogant
           claims"; "chesty as a peacock" [syn: arrogant,
           chesty, self-important]

Moby Thesaurus II by Grady Ward, 1.0:

145 Moby Thesaurus words for "arrogant":
   absolute, absolutist, absolutistic, affected, arbitrary,
   aristocratic, artificial, assuming, assured, audacious,
   authoritarian, authoritative, autocratic, big, bloated, boastful,
   bold, bossy, brash, brassy, brazen, bumptious, cavalier,
   challenging, cheeky, clannish, cliquish, cocksure, cocky, cold,
   conceited, condescending, confident, contemptuous, contumelious,
   convinced, cool, daring, decided, defiant, defying, derisive,
   despotic, determined, dictatorial, disdainful, disregardful,
   domineering, egotistical, exclusive, familiar, feudal, forward,
   greatly daring, grinding, haughty, high and mighty,
   high-and-mighty, high-faluting, high-flown, high-handed,
   high-headed, high-nosed, highfalutin, hoity-toity, hubristic,
   imperative, imperial, imperious, impertinent, important, impudent,
   insolent, insulting, lofty, lordly, magisterial, magistral,
   mannered, masterful, monocratic, obtrusive, oppressive,
   overbearing, overconfident, overpresumptuous, overruling, oversure,
   overweening, patronizing, peremptory, persuaded, pert, poised,
   pompous, pontifical, positive, presuming, presumptuous, procacious,
   proud, puffy, purse-proud, pushy, reassured,
   regardless of consequences, repressive, saucy, scornful, secure,
   self-assertive, self-assured, self-confident, self-important,
   self-reliant, severe, showy, sneering, sniffy, snobbish, snobby,
   snooty, snotty, strict, stuck-up, stuffy, supercilious, superior,
   suppressive, sure, swaggering, toploftical, toplofty, tyrannical,
   tyrannous, unafraid, undoubting, unfaltering, unhesitating,
   unwavering, uppish, uppity, upstage, vain, withering