This is not an American expression, that's why you cannot find its meaning.
It means to stand around doing nothing without much of a care or purpose. From what I can tell, it is a British expression and nobody seems to know who or what Piffy is. The more common form of the expression is to be "sitting like Piffy on a rock bun" (a rock bun is a cake, apparently). It is similar to the American English expression, "Sitting like a bump on a log." It is often used as an expression of irritation and is related to an older expression "sitting like Patience on a monument," another phrase that has no meaning whatsoever in American English.
See the link below for possible explanations.