My Arse Meaning

(slang, Britain) Indicates incredulous disapproval, contradiction or disbelief.

Example: 2015, Colin Burrow, ‘The Empty Bath’, London Review of Books, vol. 37 no. 12:
  Does this circuitous process of misinformation display the ‘liquid clearness of an Ionian sky’? My arse it does.
  Denise Royle: Dad! Your fly hole's all undone.
  Jim Royle: Ah, the cage might be open, but the beast is asleep.
  Barbara Royle: Beast my arse. - The Royle Family