The Collaborative International Dictionary of English v.0.48:

Batman \Bat"man\ (b[add]"man or b[a^]t"man), n.; pl. Batmen
   (b[a^]t"men). [F. b[^a]t packsaddle + E. man. Cf.
   Bathorse.]
   A man who has charge of a bathorse and his load. --Macaulay.
   [1913 Webster]