The New Partridge Dictionary of Slang & Unconventional English by Tom Dalzell & Terry Victor (publ. Routledge 2005), suggests it's possible etymology derives from "Slap" - make-up or the sound of flesh on flesh.
The Oxford English Dictionary records it in 1988 questions the possibility that it derives from the Yiddish "Schlepper" meaning an unkempt, scruffy person; gossipy, dowdy.