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]