“A good ball to use in connection with the fastball is what is known in the profession as the slow ball, also known as the palm ball. This is thrown with exactly the same motion as the fastball, and is therefore a gay deceiver to all but exceptionally clever batsmen. When a pitcher desires to use the palm ball he places the horsehide in the palm of his hand and makes the same swing as for the fastball; that is, bringing the arm well back over the head, and then straight forward at full length, but puts very little effort in the delivery. The slow ball has no curve, and very often does not revolve when on its way to the plate.” Christy Mathewson, San Francisco Examiner, October 30, 1905, p8
“There is the slow ball and the fade-away and the knuckle ball and the hock ball, and the spit ball and the palm ball and the high floater and the fast fader, and the thumb jumper and the ball that changes its pace with a seeming intelligence that fools the man at the plate.” Los Angeles Times, December 4, 1923, p47
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1948. “[Ewell Blackwell demonstrates] how he holds his new pitch, the ‘Palm Ball,’ which he has added to his repertoire this year” (Baseball Magazine, June).