“Yes, that fall-away slide of Mike’s was a beaut, and no mistake….” Jim McGuire quoted in “Mike Kelley’s Wager,” Washington Star, May 3, 1908, Part 5 p4
— Describes Kelley’s slide as “going into the bag like a man shot out of a cannon, and at the same instant making a peculiar wide swerve, that would throw his body entirely out of reach of the expectant second baseman. No one was ever successful with this great slide but Kelley….”
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1910. (Baseball Magazine, May, p.72; Edward J. Nichols).