More politely known as "a pile of poo", the term "a crock of shit" derives from an ancient Roman custom that coincidentally took place in Roman times. It referred literally to a pot into which people would excrete if they were particularly bored by whichever freelance philosoper happened to be talking rubbish at the time. The Roman empire employed crock-monitors who were each assigned to a philosopher, and it was their job to monitor the pot (or crock). Should the crock become full, it would be presented to the philospher, who was obliged, by law, to announce that it bore a remarkable resembence to himself, thus proclaiming he was full of crap and was, in fact, talking a crock of shit.
"I am talking a crock of shit", Socretes 429 BC