An extremely inept or clumsy person.
A person who means well but is incompetent, ineffective, etc. and is consistently in trouble.
(informal) A perennial victim of misfortune.
Origin of sad-sack
US 1920s. Popularized by Sad Sack, a cartoon character and eponymous comic strip published originally June 1942 in Yank, the Army Weekly, a US Army publication for soldiers, and later syndicated in the US 1940s and 1950s. Presumably from vulgar “sad sack of shit"; Cartoonist Sgt. George Baker said he took from a “longer phrase, of a derogatory nature". The term originally referred to a well-meaning but inept soldier.