ground rules (1878)

“The right field fence will be rather close in, and a special ground-rule for hits in that direction will have to be made.” “The Lake-Front Ball Park: How It Was Inclosed Yesterday,” Chicago Tribune, March 18, 1878, p8

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1889.  “Under the ground rules agreed to before the game, the umpire gave Nat [Hudson] a home run.” (St. Louis Post-Dispatch, Apr. 7; Peter Morris).