The Collaborative International Dictionary of English v.0.48:

Endorse \En*dorse"\, v. t. [imp. & p. p. Endorsed; p. pr. &
   vb. n. Endorsing.] [Formerly endosse, fr. F. endosser to
   put on the back, to endorse; pref. en- (L. in) + dos back, L.
   dorsum. See Dorsal, and cf. Indorse.]
   Same as Indorse.
   [1913 Webster]

   Note: Both endorse and indorse are used by good writers; but
         the tendency is to the more general use of indorse and
         its derivatives indorsee, indorser, and indorsement.
         [1913 Webster]