“‘Kid’ Peoples, at short, handles ball as if it were no effort at all … He is a good line hitter.” Sacramento Daily Record-Union, March 12, 1891, p3
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1928. “[Lou Gehrig is] what we call a ‘line hitter’” (George Herman Ruth, Babe Ruth’s Own Book of Baseball, p. 155; Peter Morris).