The Collaborative International Dictionary of English v.0.48:

Mortal \Mor"tal\, a. [F. mortel, L. mortalis, from mors, mortis,
   death, fr. moriri 8die; akin to E. murder. See Murder, and
   cf. Filemot, Mere a lake, Mortgage.]
   1. Subject to death; destined to die; as, man is mortal.
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   2. Destructive to life; causing or occasioning death;
      terminating life; exposing to or deserving death; deadly;
      as, a mortal wound; a mortal sin.
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   3. Fatally vulnerable; vital.
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            Last of all, against himself he turns his sword, but
            missing the mortal place, with his poniard finishes
            the work.                             --Milton.
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   4. Of or pertaining to the time of death.
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            Safe in the hand of one disposing Power,
            Or in the natal or the mortal hour.   --Pope.
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   5. Affecting as if with power to kill; deathly.
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            The nymph grew pale, and in a mortal fright.
                                                  --Dryden.
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   6. Human; belonging to man, who is mortal; as, mortal wit or
      knowledge; mortal power.
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            The voice of God
            To mortal ear is dreadful.            --Milton.
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   7. Very painful or tedious; wearisome; as, a sermon lasting
      two mortal hours. [Colloq.] --Sir W. Scott.
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   Mortal foe, Mortal enemy, an inveterate, desperate, or
      implacable enemy; a foe bent on one's destruction.
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WordNet (r) 3.0 (2006):

mortal enemy
    n 1: an enemy who wants to kill you