This term meaning to be mixed up has nothing to do with rhyming slang but is very old and probably derives from a dice game. See http://www.quinion.com/words/qa/qa-six1.htm
Sorry geofbob nothing to do with dice. This comes from the medieval guild of london whenthe merchant tailors and skinners couldn't decide who should be 6 and who 7. so they took it in turns which they still do to this day. Source-Bloomsbury reference
Sorry, sft42, but I still go with the dice explanation. Why should some local tiff between two London guilds get into the national language? In any case, the term was originally used - eg by Chaucer in 1374 - to mean to hazard a lot on a little, so the connection with gambling was fairly clear. Only later did it evolve into today's meaning of carelessness and confusion.