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.