Rhyming slang is often used to cover up even more offensive words, the oft used "berk" as an insult derives from the rhyming slang of "The Berkeley Hunt". (you can work that out for yourself, I'm too polite!).
If you haven't already guessed cobbler's awls is rhyming slang for balls, which can be used to mean men's testicles ("he got a kick in the cobbler's") or more commonly for nonsense or rubbish.
Let's use it in a sentence:
"All that talk of weapons of mass destruction turned out to be a load of old cobbler's" which translates to "Politicians talk a lot of crap".