‘porky’ (rhyming slang for ‘lie’)

In British English, the noun porky (also porkie) is short for porky pie (also porkie pie), which is an alteration of pork pie, rhyming slang 1 for the noun lie, denoting an untrue or deceptive statement deliberately used to mislead.

1 Rhyming slang is a type of slang that replaces words with rhyming words or phrases, typically with the rhyming element omitted. For example, in the phrase not to have a scooby, meaning not to have a cluescooby is:
– short for Scooby Doo, the name of a cartoon dog which features in several U.S. television series and films;
– rhyming slang for clue.

The plural noun porkies occurs, for example, in Steven Poole’s word of the week, published in The Guardian (London and Manchester, England) of Thursday 24th June 2021:

Brexit porkies: the etymology of ‘sausage’, star of the latest trade row
As a correspondent, Boris Johnson 2 cooked up stories about European food regulation; as the PM, he is battling to avoid a trade war over sausages
Are you getting hot under the collar about chilled meats? Boris Johnson, who made his mark as a journalist inventing stories about European food regulation, is now outraged that shipping sausages from the UK to Northern Ireland requires paperwork, which is what he agreed six months ago. As you might say if feeling charitable, what a silly sausage.

2 The British politician, author and former journalist Boris Johnson (born 1964) has served as Prime Minister of the United Kingdom and Leader of the Conservative Party since July 2019.

The earliest occurrences that I have found of the nouns pork pie, porky pie (also porkie pie) and porky (also porkie) denoting a lie are as follows, in chronological order:

1-: From A Load of Cockney Cobblers (Edinburgh & London: Johnston & Bacon, 1973), by Bob Aylwin:

Pork Pie A lie

2-: From this advertisement, published in The Standard (London, England) of Wednesday 20th April 1983:

We don’t tell
Porky Pies
Dickie Dirts 3
DIRTY DICKS 4
GENUINE PERFECT LEVI’S JEANS £10.99
OPEN SUNDAYS 10 am—10 pm
Same times every day of the week
58a WESTBOURNE GROVE, LONDON W2. 01-229 1466/7765

3 Dickie Dirt is rhyming slang for shirt.
4 Dirty Dicks was the name of a clothes shop in Westbourne Grove, London.

3-: From the Ealing Gazette (London, England) of Friday 1st June 1984:

Check out your lights

ELECTRONIC wizardry, computerised gadgets and silicon chips in the engine are all the rage with modern motorists.
Bleepers and synthesisers and things that go flash in the night are all designed to ease the burden on the driver and tell him when he is running out of petrol, oil, water, brake fluid, etc.
But what happens when things start flashing that should not and that sultry voice synthesiser starts telling pork pies.
On two occasions, in different cars, I have had warning lights telling me my oil and coolant levels were low when they were clearly not.

4-: From Did she fall or was she pushed?, an article by Irene McManus about “Manchester’s newest and “friendliest” marriage bureau, Knights Introductions”, published in The Guardian (London and Manchester, England) of Wednesday 13th February 1985—in the following, Irene McManus is quoting Kevin Brown, the head of Knights Introductions:

Some of the men as well as the women, “tell little porky pies about their age.”

5-: From Strained Relations, an episode of the television sitcom Only Fools and Horses…., written by the British scriptwriter John Sullivan (1946-2011), first broadcast on BBC One on Thursday 28th February 1985—source: Only Fools And Horses Quotes:

Del—Listen Rodney, that bloke has been in shark-infested seas, right, he’s been attacked by kamikaze pilots, and blown up more times than a beach-ball! One night in a dosshouse ain’t gonna do him any harm, is it?
Rodney—You don’t believe all them stories do you?
Del—What? Do you reckon they’re porkies?
Rodney—Well of course they are! I didn’t wanna say nothing ’cos, you know, he’s a proud man!

6-: From The Club, by Gill Pringle, published in The Mirror (London, England) of Tuesday 13th August 1985:

WANT to know if your partner is telling porkie pies? Paul Ekman, author of Telling Lies, has plenty of advice on telltale signs. He says liars’ voices rise and get louder, their smile comes and goes rapidly and they raise their eyebrows . . . so now you know.

7-: From the transcript of the testimony of William Morrison, the plaintiff in a trial at the High Court in Ayr, published in the Ayrshire Post (Ayr, Ayrshire, Scotland) of Friday 2nd May 1986:

“He kept accusing me of telling ‘porky pies’ (lies).”

8-: From John Blake’s White Hot Club, published in the Daily Mirror (London, England) of Wednesday 5th November 1986:

Porky pies of the stars

THE stars have revealed the biggest lies they have ever told. And some of them are real whoppers!
Neil (Pet Shop Boys): “I wouldn’t want my mother to read this but when I was a little boy I set the garage alight.”
[&c.]

9-: From the Daily Mirror (London, England) of Friday 11th December 1987:

HE’S A WALLY WHO TELLS PORKY PIES TO GET NOTICED, SAYS MUM

Joyful mother Sylvia Bishop said after her son Russell’s acquittal: “It’s wonderful. I always knew he was innocent.”
[…]
When he was arrested, she said: “They have got a wally.
“He does tell porkies (lies). He does it for attention. But he’s never hurt anybody by his porky pies, not in his life.”