The Collaborative International Dictionary of English v.0.48:

Peremptory \Per"emp*to*ry\, a. [L. peremptorius destructive,
   deadly, decisive, final: cf. F. p['e]remptorie. See
   Perempt.]
   1. Precluding debate or expostulation; not admitting of
      question or appeal; positive; absolute; decisive;
      conclusive; final.
      [1913 Webster]

            Think of heaven with hearty purposes and peremptory
            designs to get thither.               --Jer. Taylor.
      [1913 Webster]

   2. Positive in opinion or judgment; decided; dictatorial;
      dogmatical.
      [1913 Webster]

            Be not too positive and peremptory.   --Bacon.
      [1913 Webster]

            Briefly, then, for we are peremptory. --Shak.
      [1913 Webster]

   3. Firmly determined; unawed. [Poetic] --Shak.
      [1913 Webster]

   Peremptory challenge (Law) See under Challenge.

   Peremptory mandamus, a final and absolute mandamus.

   Peremptory plea, a plea by a defendant tending to impeach
      the plaintiff's right of action; a plea in bar.
      [1913 Webster]

   Syn: Decisive; positive; absolute; authoritative; express;
        arbitrary; dogmatical.
        [1913 Webster]

WordNet (r) 3.0 (2006):

peremptory
    adj 1: 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]
    2: not allowing contradiction or refusal; "spoke in peremptory
       tones"; "peremptory commands"
    3: putting an end to all debate or action; "a peremptory decree"

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "peremptory":
   absolute, absolutist, absolutistic, admitting no exception,
   all-out, arbitrary, aristocratic, arrogant, authoritarian,
   authoritative, autocratic, bigoted, binding, bossy, categorical,
   certain, clear, commanding, compelling, complete, compulsory,
   conceited, conclusive, de rigueur, decided, decisive, decretal,
   decretive, decretory, definite, definitive, despotic, determinate,
   dictated, dictating, dictatorial, directive, doctrinaire,
   doctrinarian, dogmatic, dogmatizing, domineering, downright,
   emphatic, entailed, entire, explicit, express, feudal, final, firm,
   fixed, flat, flat-out, global, grinding, hard-and-fast,
   high-handed, imperative, imperial, imperious, implicit, imposed,
   inappealable, incontrovertible, indisputable, insistent,
   instructive, irrefutable, irrevocable, jussive, lordly,
   magisterial, magistral, mandated, mandating, mandatory, masterful,
   monocratic, must, necessary, obligating, obligatory, obstinate,
   opinionated, opinionative, opinioned, oppressive, oracular,
   out-and-out, outright, overbearing, overruling, perfect,
   pontifical, positive, positivistic, preceptive, preclusive,
   prescript, prescriptive, pronunciative, repressive, required,
   round, self-opinionated, self-opinioned, severe, straight,
   straight-out, strict, suppressive, total, tyrannical, tyrannous,
   ultimate, uncircumscribed, uncompromising, unconditional,
   unconditioned, undoubting, unequivocal, unhampered, unhesitating,
   unlimited, unmistakable, unmitigated, unqualified, unquestioning,
   unreserved, unrestricted, unwaivable, utter, whole, without appeal,
   without exception, without reserve



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

PEREMPTORY. Absolute; positive. A final determination to act without hope of
renewing or altering. Joined to a substantive, this word is frequently used
in law; as peremptory action; F. N. B. 35, 38, 104, 108; peremptory nonsuit;
Id. 5, 11; peremptory exception; Bract. lib. 4, c. 20; peremptory
undertaking; 3 Chit. Pract. 112, 793; peremptory challenge of jurors, which
is the right to challenge without assigning any cause. Inst. 4, 13, 9 Code,
7, 50, 2; Id. 8, 36, 8; Dig. 5, 1, 70 et 73.