aspirin (1939)

“And just imagine being the batter out there under the arcs with Feller, whose control is none too good and whose blazing speed makes his high hard one look like an aspirin in the daytime, throwing that left foot up in your face, rarin’ back and pumping one in the general direction of the plate.” Bob Ray, “The Sports X-Ray,” Los Angeles Times, February 5, 1939, Part II,p12

No earliest use given in 2009 Dickson Baseball Dictionary