sworn enemies

from The Collaborative International Dictionary of English v.0.48

Sworn \Sworn\,
   p. p. of {Swear}.
   [1913 Webster]

   {Sworn brothers}, originally, companions in arms who took an
      oath to share together good and bad fortune; hence,
      faithful friends.

   {Sworn enemies}, determined or irreconcilable enemies.

   {Sworn friends}, close friends.
      [1913 Webster]