leave the yard (1977)

“[George] Brett said he didn’t know if the ball would leave the yard or not. ‘That’s the toughest part of the park to get one out, you know,’ he said.” Charlie Smith, “Royals Clip White Sox, 6-3,” Wichita Eagle and Beacon, August 7, 1977, p1D

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1986.  “Today, [when hitting a home run] you go to the suburbs, just go deep or leave the yard.” (R.J. Reynolds, quoted by Bob Hertzel, Scripps Howard News Service, Sept. 17).