“Whether he gave them in curves or out curves, or varied the programme with rising curves or drop curves, the event was the same—they lashed frantically into emptiness, or sent the ball obligingly into the hands of some ambitious fielder.” Buffalo Express, September 29, 1879, p4
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1886. “He possesses a drop curve that proves a puzzler” (The Sporting News, May 17; David Shulman).