The Collaborative International Dictionary of English v.0.48:

Imperious \Im*pe"ri*ous\, a. [L. imperiosus: cf. F.
   imp['e]rieux. See Imperial.]
   1. Commanding; ascendant; imperial; lordly; majestic. [Obs.]
      "A vast and imperious mind." --Tilloison.
      [1913 Webster]

            Therefore, great lords, be, as your titles witness,
            Imperious.                            --Shak.
      [1913 Webster]

   2. Haughly; arrogant; overbearing; as, an imperious tyrant;
      an imperious manner.
      [1913 Webster]

            This imperious man will work us all
            From princes into pages.              --Shak.
      [1913 Webster]

            His bold, contemptuous, and imperious spirit soon
            made him conspicuous.                 --Macaulay.
      [1913 Webster]

   3. Imperative; urgent; compelling.
      [1913 Webster]

            Imperious need, which can not be withstood.
                                                  --Dryden.

   Syn: Dictatorial; haughty; domineering; overbearing; lordly;
        tyrannical; despotic; arrogant; imperative;
        authoritative; commanding; pressing.

   Usage: Imperious, Lordly, Domineering. One who is
          imperious exercises his authority in a manner highly
          offensive for its spirit and tone; one who is lordly
          assumes a lofty air in order to display his
          importance; one who is domineering gives orders in a
          way to make others feel their inferiority.
          [1913 Webster]

WordNet (r) 3.0 (2006):

imperious
    adj 1: having or showing arrogant superiority to and disdain of
           those one views as unworthy; "some economists are
           disdainful of their colleagues in other social
           disciplines"; "haughty aristocrats"; "his lordly manners
           were offensive"; "walked with a prideful swagger"; "very
           sniffy about breaches of etiquette"; "his mother eyed my
           clothes with a supercilious air"; "a more swaggering mood
           than usual"- W.L.Shirer [syn: disdainful, haughty,
           imperious, lordly, overbearing, prideful,
           sniffy, supercilious, swaggering]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "imperious":
   U, absolute, absolutist, absolutistic, acute, arbitrary,
   aristocratic, arrogant, authoritarian, authoritative, autocratic,
   binding, bossy, clamorous, coactive, compelling, compulsatory,
   compulsive, compulsory, constraining, critical, crucial, crying,
   de rigueur, despotic, dictatorial, domineering, driving, dynastic,
   elitist, exigent, feudal, grinding, heavy-handed, high and mighty,
   high-handed, high-pressure, high-priority, hubristic, imperative,
   imperatorial, imperatorious, imperial, insistent, instant,
   irresistible, kinglike, kingly, lordly, magisterial, magistral,
   majestic, mandatory, masterful, monarchal, monarchial, monarchic,
   monocratic, must, necessary, obligatory, oppressive, overbearing,
   overruling, peremptory, pivotal, pressing, princelike, princely,
   purple, queenlike, queenly, regal, repressive, required,
   restraining, royal, severe, sovereign, strict, stringent,
   suppressive, tyrannical, tyrannous, urgent