Shit sandwich is a metaphor for an unpleasant situation that one is forced to endure. The normal usage is in the phrase "eating a shit sandwich". The metaphor follows from the idea that eating a sandwich composed of feces would be highly unpleasant.
An expansion on this saying is "The less bread you have, the more shit you have to taste", where bread is a metaphor for money; the saying means that those with less money will have more unpleasant experiences.
The expression has found some degree of favor within the US Armed Forces, along with several other "off-color" phrases. It is not uncommon in United States slang in some parts of the country.
See also: SNAFU
