“In the afternoon at 3:30 o’clock, Arthur Irwin, short-stop and captain of the Philadelphia League club, in the capacity of umpire, called ‘Batter up!,’ and Reccius pitched the first ball for the Louisville team for 1886.” The Courier-Journal [Louisville] March 15, 1886, p4
“After an interval of a month since the last Tri State League game was played here, the old familiar sounds of ‘play ball,’ ‘now you’re off,’ ‘batter up,’ etc., will be heard once again at Pastime Park…” Canton (OH) Repository June 8, 1888 [Richard Hershberger]
No earliest use given in 2009 Dickson Baseball Dictionary
