A batman (or batwoman) is a soldier or airman assigned to a commissioned officer as a personal servant. Before the advent of motorized transport, an officer's batman was also in charge of the officer's "bat-horse" that carried the pack saddle with his officer's kit during a campaign.
The term is derived from the obsolete bat, meaning "pack saddle" (from French bāt, from Old French bast, from Late Latin bastum), and man.
A batman's duties often include:
acting as a "runner" to convey orders from the officer to subordinates
maintaining the officer's uniform and personal equipment as a valet
driving the officer's vehicle, sometimes under combat conditions
acting as the officer's bodyguard in combat
other miscellaneous tasks the officer does not have time or inclination to do
The action of serving as a batman was referred to as "batting".