[1] : 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.
      1913 webster

   2. destructive to life; causing or occasioning death;
      terminating life; exposing to or deserving death; deadly;
      as, a mortal wound; a mortal sin.
      1913 webster

   3. fatally vulnerable; vital.
      1913 webster

            last of all, against himself he turns his sword, but
            missing the mortal place, with his poniard finishes
            the work.                             --milton.
      1913 webster

   4. of or pertaining to the time of death.
      1913 webster

            safe in the hand of one disposing power,
            or in the natal or the mortal hour.   --pope.
      1913 webster

   5. affecting as if with power to kill; deathly.
      1913 webster

            the nymph grew pale, and in a mortal fright.
                                                  --dryden.
      1913 webster

   6. human; belonging to man, who is mortal; as, mortal wit or
      knowledge; mortal power.
      1913 webster

            the voice of god
            to mortal ear is dreadful.            --milton.
      1913 webster

   7. very painful or tedious; wearisome; as, a sermon lasting
      two mortal hours. colloq. --sir w. scott.
      1913 webster

   mortal foe, mortal enemy, an inveterate, desperate, or
      implacable enemy; a foe bent on one's destruction.
      1913 webster
see also:
murder filemot mere mortgage mortal foe mortal enemy 

[2] : WordNet (r) 2.0

mortal enemy
     n : an enemy who wants to kill you

Enemy >> E Words
Enemy, definition of term: Enemy
enemy_pag1.html

Mortal >> M Words
Mortal, definition of term: Mortal
mortal_pag1.html

Mortal Sin >> M Words
Mortal Sin, definition of term: Mortal Sin
mortal+sin_pag1.html