“[Dick] Bartell, the right-handed batter, wasn’t showing the proper respect for [Lon] Warneke, the right-handed pitcher. So Warneke threw a knock-down pitch.” J. Roy Stockton, St. Louis Post-Dispatch, May 21, 1937, p20.
“Dizzy [Dean] then whipped across his ‘knock-a-down’ pitch—a high, fast one close to the chin.” Sid Keener, St. Louis Star-Times, April 3, 1937, p5.
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1962. “There’s the brushback, and there’s the knockdown pitch” (The Saturday Evening Post, June 30; David Shulman).