This term comes from cockney rhyming slang,^[1] a form of communication originated in old east London by merchants to communicate with each other in a way that is disguised and incomprehensible to outsiders. Unlike most rhyming slang expressions, it is still in semi-popular use both in London and outside.

The expression is a synonym for ‘lies’. Note how the second word ‘pies’ rhymes directly with ‘lies’.  As such when you hear it in use, even if you aren’t familiar with expression you can often tell what is being said by the rhyme and the context it is being used in.

For example, ‘Don’t listen to him he’s telling pork pies.’