Guess Hitter (1921)

“In hitting Ward far surpasses Rawlings across the season; … Both are ‘guess’ hitters but Ward is liable to pole a long one at any time.” Wilkes-Barre Times Leader, October 4, 1921 p14

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1928. “No ‘guess’ hitter ever makes a good hitter” (George Herman Ruth, Babe Ruth’s Own Book of Baseball, 1928, p. 213; Peter Morris).