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.
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            Think of heaven with hearty purposes and peremptory
            designs to get thither.               --Jer. Taylor.
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   2. Positive in opinion or judgment; decided; dictatorial;
      dogmatical.
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            Be not too positive and peremptory.   --Bacon.
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            Briefly, then, for we are peremptory. --Shak.
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   3. Firmly determined; unawed. [Poetic] --Shak.
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   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.
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   Syn: Decisive; positive; absolute; authoritative; express;
        arbitrary; dogmatical.
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Bouvier's Law Dictionary, Revised 6th Ed (1856):

PEREMPTORY PLEA, pleading. A plea which denies the plaintiff's cause of
action. 3 Bouv. Inst. n. 2891. Vide Plea.