Piffy on a rock bun

what does 'Piffin on a rock bun' mean - my mother just used this expression that I remember hearing as a child - spelling may be wrong?

"Heard in phrases expressing upset that one has been ignored or left out of an activity. Often heard shortened to "like piffy on a rock", and also just "like piffy". Etymology unknown.

"She forgot to tell me the meal had been cancelled, and left me standing in the rain like piffy."[North-west use]