“When Sain joined the Twins in 1965, Kaat immediately started using Sain’s fast curve, instead of the slider. ‘I call it a slurve,’ said Jim. ‘It’s part slider and part curve.'” Jim Kaat quoted in Max Nichols, “Planning, Organization, Efficiency: Kaat Reaps Rewards, Seek No. 20,” Minneapolis Star, August 26, 1966, p7B
“‘His pet pitch, one he has taught to just about every one of the boys on our staff, is what he calls a ‘slurve.’ It is a fastball and it is a curve — but it is not a slider, which has a flatter trajectory.'” Jim Kaat, quoted in Clark Stoppels, “The View from Here,” Grand Rapids (MI) Press, October 10, 1966, p39
NOTE: Although the earlier quote from Kaat says “slurve” is what he calls the pitch taught to him by pitching coach Johnny Sain, the later quote from Kaat says it is Sain’s name for the pitch.
No earliest use given in 2009 Dickson Baseball Dictionary