British phrase to signify "I don't believe you", as in:

Person A: "I saw Elvis in the pub the other night!"
Person B: "Yeah, my arse!"

Also the catchphrase of patriarch Jim Royle in family "comedy" The Royle Family, to show he disapproves of (usually) something he has seen on the TV. Personally I don't care much for the actor who plays him as he was in the NF until the age of 35, but there ya go.