Pig Slap Up Meal Excellent Large Cheap Fatty  (Urban). 

A very cheap, and fatty, meal.  (urban). 

The expression goes back to the time of Charles Dickens, when it was a "slap-bang" meal, derived from cheap eating houses, where one slapped one's money down as the food was banged on the table. Why "down" has turned to "up" is probably another example of language evolution, in much the same way as "to be sold a pig in a poke" has come to mean that one has been cheated, whereas, in reality, the reason for going to a medieval market was often to buy the pig and not to be "sold a pup"! - Brewer's.