From the U.S. Army's
Airborne School at Fort Benning, Georgia.
To make a
soup sandwich you get some bread, then take a big ladle of soup, pour it on the bread, take another piece of bread, put it on top, and try to eat it.
Of cource it falls apart almost immediately.
In the
Military a person who is a 'soup sandwich' could be any combination of disorganized, late, unprepared, or
unmotivated.