An awkward, unfortunate, harried, and maladjusted person; eightball, schlemazel: nevertheless stuck with an inexplicable sad sack of a leading man
[Students 1920s+ & esp WWII armed forces; fr the unflattering image of a human being as primarily a container for feces]
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with sad-sack
A singularly inept person, as in Poor George is a hopeless sad sack. This term alludes to a cartoon character, Sad Sack, invented by George Baker in 1942 and representing a soldier in ill-fitting uniform who failed at whatever he tried to do. It was soon transferred to clumsily inept civilians.