Cobbler's awls -- > balls
This is from world wide words, interesting site.
http://www.worldwidewords.org/qa/qa-cob1.htm
"The source is the phrase cobbler’s awl.
An awl is an ancient pointed tool for making holes in things, the most characteristic tool of any leatherworker; it was an essential part of the shoemaker’s kit, since he was forever piercing leather to sew pieces together. So a cobbler’s awl was as characteristic of his trade as his last, or foot-shaped anvil.
The rhyming slang linked cobbler’s awls with balls, that is, testicles. As was usual with such rhyming slang phrases, the first word later appeared on its own as a kind of half-disguised code, so cobblers! came to be used in the same way as balls!, as an exclamation of derision or disbelief, suggesting something was rubbish or nonsense."