“…. Pickering, the club swinger, pounded out a good imitation of a Baltimore chop, as the ball jumped over Killen’s head and seemed safe.” The Pittsburgh Press, October 3, 1897, p13
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1905. “[Joe] Kelley netted a high bouncing bingle in old-fashioned Baltimore chop.” Boston Daily, June 16, 1905.