A [**sad sack**](https://www.merriam-webster.com/dictionary/sad%20sack) is *a blundering, inept person*. 

MW defines it as "US, informal", and says the first known use with this meaning is from 1943. However, [etymonline](https://www.etymonline.com/word/sad#etymonline_v_22587) says:

> *sad sack* is 1920s, popularized by World War II armed forces
> (specifically by cartoon character invented by Sgt. George Baker,
> 1942, and published in U.S. Armed Forces magazine "Yank"), probably a
> euphemistic shortening of common military slang phrase *sad sack of
> shit*.

I believe the term comes from a military context, a soldier who can't seem to do anything correctly.

My suspicion is that some drill sargent called his recruit a sack of shit. Then found the poetic turn in sad sack of shit, which was cleaned up for Folks Back Home as **sad sack**.

The definitions in the Urban Dictionary seem to turn the meaning more toward a depressed or depressing person. But the original context, I believe, used sad in the sense of pitiful.