Green’s Dictionary of Slang

(orig. US) a miserable, pessimistic, morale-lowering person.

1943		in G. Legman Limerick (1953) 183: Maggie is such a sad sack of shit / That no one will tickle her tit.
1961		(con. early 1950s) J. Peacock Valhalla 342: He a sad sack of shit.
1978		S. King Stand (1990) 239: Some sad sack of shit like you.
1984		J. Dailey Silver Wings (1985) 125: I’m not going to have you drag us all down because you look like a sad sack of shit.
1992		Randolph & Legman Ozark Folksongs and Folklore I 447: Uncle Joe [...] is a thoroughgoing loser, or Ozark ‘Schlemihl,’ like Peter Schlemihl, a German folk-fool who sells his shadow to the Devil and thereafter has a lifetime of bad luck; his name now is a synonym for a ‘sad sack (of shit)’.
2000		C. Cook Robbers (2001) 274: Pardner, you looking at a sad sack of shit.