where does the term "bat man" in the English army come from

Hello.

From the American Heritage Dictionary of the English Language:

"bat·man
NOUN:  A British military officer's orderly.
ETYMOLOGY: Obsolete bat, packsaddle (from French bāt, from Old French
bast, from Late Latin bastum) + man"
source: Bartleby.com
http://www.bartleby.com/61/45/B0114500.html

Additional source:

" An officer’s batman has nothing to do with a bat or the officer’s
batting. This word goes back to the times when army officers travelled
on horseback and their baggage was also transported by horses. Batman
comes from the Latin bastum (packsaddle), a word which French borrowed
in the form of bat or bast. English borrowed it subsequently, making
it batman."
source: tribunemedia.com
http://www.tribuneindia.com/2001/20010623/windows/roots.htm

search strategy:
bartleby.com, dictionary, batman

I hope this helps.