A term used in the UK when someone is desperately trying to convince you that their piece of information or version of events is correct to gain your trust but you know that they're blatantly lying through their teeth. Or someone is trying to sell you the latest designer object (watch, trainers, handbag etc.) and you can clearly see it's a fake. It's as genuine as a 40 pence piece. There is no such thing as a 40 pence piece in UK currency so if someone tried to give you one you'd know it wasn't real money.
A woman looking at handbags on Ebay: "oooo have you seen this lovely designer handbag it's only £10"
Friend: " narr mate it's a fake"
Woman: " it looks genuine to me"
Friend: " genuine! It's about as genuine as a 40 pence piece"
Or
A man after been told someone unreliable information: " rubbish! what a load of old tosh"
The informer: " narr mate it's absolutely genuine"
Man: "yeah about as genuine as a 40 pencepiece"
Friend: " narr mate it's a fake"
Woman: " it looks genuine to me"
Friend: " genuine! It's about as genuine as a 40 pence piece"
Or
A man after been told someone unreliable information: " rubbish! what a load of old tosh"
The informer: " narr mate it's absolutely genuine"
Man: "yeah about as genuine as a 40 pencepiece"
by hteb78 January 23, 2024
Get the As genuine as a 40 pence piecemug. 
by beentherdonethat September 3, 2014
Get the 40 poppermug. Look at the ass on her, I'd like to wear it as a hat, damn that makes hotter than a dog with 40 dicks.
by Fa Fa January 2, 2008
Get the Dog with 40 dicksmug. 
If you see 40■4 written on the palm of someone's hand it means thay are willing to stand up against the government to protect our constitutional rights if it is necessary
by jj101202 August 16, 2021
Get the 40■4mug. by JennyFyre June 3, 2021
Get the Front 40mug.