to signify "I don't believe you", as in:
: "I saw Elvis
in the pub
the other night!"
: "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
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.