The Collaborative International Dictionary of English v.0.48:

Magisterial \Mag`is*te"ri*al\, a. [L. magisterius magisterial.
   See Master.]
   1. Of or pertaining to a master or magistrate, or one in
      authority; having the manner of a magister; official;
      commanding; authoritative. Hence: Overbearing;
      dictatorial; dogmatic.
      [1913 Webster]

            When magisterial duties from his home
            Her father called.                    --Glover.
      [1913 Webster]

            We are not magisterial in opinions, nor,
            dictator-like, obtrude our notions on any man. --Sir
                                                  T. Browne.
      [1913 Webster]

            Pretenses go a great way with men that take fair
            words and magisterial looks for current payment.
                                                  --L'Estrange.
      [1913 Webster]

   2. (Alchem. & Old Chem.) Pertaining to, produced by, or of
      the nature of, magistery. See Magistery, 2.
      [1913 Webster]

   Syn: Authoritative; stately; august; pompous; dignified;
        lofty; commanding; imperious; lordly; proud; haughty;
        domineering; despotic; dogmatical; arrogant.

   Usage: Magisterial, Dogmatical, Arrogant. One who is
          magisterial assumes the air of a master toward his
          pupils; one who is dogmatical lays down his positions
          in a tone of authority or dictation; one who is
          arrogant insults others by an undue assumption of
          superiority. Those who have long been teachers
          sometimes acquire, unconsciously, a manner which
          borders too much on the magisterial, and may be
          unjustly construed as dogmatical, or even arrogant.
          [1913 Webster]

WordNet (r) 3.0 (2006):

magisterial
    adj 1: of or relating to a magistrate; "official magisterial
           functions"
    2: offensively self-assured or given to exercising usually
       unwarranted power; "an autocratic person"; "autocratic
       behavior"; "a bossy way of ordering others around"; "a rather
       aggressive and dominating character"; "managed the employees
       in an aloof magisterial way"; "a swaggering peremptory
       manner" [syn: autocratic, bossy, dominating, high-and-
       mighty, magisterial, peremptory]
    3: used of a person's appearance or behavior; befitting an
       eminent person; "his distinguished bearing"; "the monarch's
       imposing presence"; "she reigned in magisterial beauty" [syn:
       distinguished, grand, imposing, magisterial]

Moby Thesaurus II by Grady Ward, 1.0:

163 Moby Thesaurus words for "magisterial":
   Daedalian, U, absolute, absolutist, absolutistic, accepted, adept,
   adroit, approved, apt, arbitrary, arch, aristocratic, arrogant,
   artistic, august, authentic, authoritarian, authoritative,
   autocratic, banner, bloated, bossy, bravura, brilliant, capital,
   cardinal, cathedral, central, champion, chief, clean, clever,
   coordinated, courtly, crack, crackerjack, crowning, cunning, cute,
   daedal, deft, despotic, dexterous, dextrous, dictatorial,
   dignified, diplomatic, disdainful, dogmatic, dominant, domineering,
   elitist, ex cathedra, excellent, expert, fancy, feudal, first,
   focal, foremost, good, goodish, graceful, grand, grave, great,
   grinding, handy, headmost, hegemonic, high and mighty, high-handed,
   hubristic, imperative, imperial, imperious, important, imposing,
   ingenious, insolent, judicative, judicatorial, judicatory,
   judicial, judiciary, jurisdictive, kingly, leading, lordly,
   magistral, main, majestic, master, masterful, masterly, monocratic,
   neat, no mean, noble, official, oppressive, overbearing,
   overruling, paramount, peremptory, politic, pontifical,
   predominant, preeminent, preponderant, prevailing, primal, primary,
   prime, princely, principal, professional, proficient, puffy,
   queenly, quick, quite some, ranking, ready, received, regal,
   repressive, resourceful, royal, ruling, sedate, self-important,
   severe, skillful, slick, sober, solemn, some, sovereign, standard,
   star, stately, statesmanlike, statuesque, stellar, strict, stuffy,
   stylish, supercilious, supereminent, suppressive, tactful,
   the compleat, the complete, topflight, tyrannical, tyrannous,
   venerable, virtuoso, well-done, workmanlike, worthy