A straightforward task that can easily be accomplished.
This is a vulgar variant of 'piece of cake'. It is of UK origin and the first records of it are from the RAF during WWII. Eric Partridge, in his A Dictionary of Slang and Unconventional English, Edition 3, 1945, lists it like this:
"Piece of piss, a ‘piece of cake’... R.A.F.: 1940+."