Definitions for sad sack
Random House Webster's College Dictionary
a pathetically inept person.
Origin of sad sack:
after the cartoon character created in 1942 by U.S. cartoonist George Baker (1915–75)
bungler, blunderer, fumbler, bumbler, stumbler, sad sack, botcher, butcher, fuckup(noun)
someone who makes mistakes because of incompetence
An incompetent or inept person.
A perennial victim of misfortune.