Source : Webster's Revised Unabridged Dictionary (1913)

Oneself \One`self"\, pron.
   A reflexive form of the indefinite pronoun one. Commonly
   writen as two words, one's self.

         One's self (or more properly oneself), is quite a
         modern form. In Elizabethan English we find a man's
         self=one's self.                         --Morris.