Bullpen Session (1933)

“Van Mungo, after a slow start and a bull-pen session early this year, pitched consistently good ball ….” Harold Parrott, Brooklyn Daily Eagle, September 5, 1933, p19

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1999. “[Rick] Ankiel is so demanding of himself that he seethes when he doesn’t throw well in bullpen sessions between starts” (Tom Verducci, Sports Illustrated, June 28, p. 66).