A 'SHIT SANDWICH' is a method of delivering some
bad news, advice, or a rejection of someone's idea(s).
It is designed to make the bad news, advice or rejection more
palatable, easier to take, and/or easier on the receiving party.
It is used to ensure the the person hearing the bad news, advice or rejection actually listens to what is being said so they can eat and
digest it...
You want someone to eat the 'Shit' (bad news), you serve it between two lovely slices of bread.. hence 'Shit Sandwich'