'Mortal enemy' definitions:

Definition of 'mortal enemy'

noun

An enemy who wants to kill you

From: GCIDE

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]