now don't tell any porky pies (lies, so where does this originate from?

pork pies = porkies = porky pies = Lies
is one of many Rhyming slang phrases (usually of Cockney -London East End- origin) to become assimilated into (mainly UK and Aussie) English. Do an AWAD search on Cockney in the last 3 months and you may find lots more.

Rod