资料来源 : Webster's Revised Unabridged Dictionary (1913)

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.

   2. Destructive to life; causing or occasioning death;
      terminating life; exposing to or deserving death; deadly;
      as, a mortal wound; a mortal sin.

   3. Fatally vulnerable; vital.

            Last of all, against himself he turns his sword, but
            missing the mortal place, with his poniard finishes
            the work.                             --Milton.

   4. Of or pertaining to the time of death.

            Safe in the hand of one disposing Power, Or in the
            natal or the mortal hour.             --Pope.

   5. Affecting as if with power to kill; deathly.

            The nymph grew pale, and in a mortal fright.
                                                  --Dryden.

   6. Human; belonging to man, who is mortal; as, mortal wit or
      knowledge; mortal power.

            The voice of God To mortal ear is dreadful.
                                                  --Milton.

   7. Very painful or tedious; wearisome; as, a sermon lasting
      two mortal hours. [Colloq.] --Sir W. Scott.

   {Mortal foe}, {Mortal enemy}, an inveterate, desperate, or
      implacable enemy; a foe bent on one's destruction.

资料来源 : WordNet®

mortal enemy
     n : an enemy who wants to kill you