“‘Tis said the Moren, the Philadelphia pitcher, throws the only knuckle curve known.” The Sun (NY), May 28, 1907, p5
See also Knuckleball, Knuckle Ball, and Knuckler
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1907. “Pitcher Morgan of the Philadelphia club has a new curve that he calls a ‘knuckle curve’” (Lansing [Mich.] Journal, June 5; Peter Morris).
