Green’s Dictionary of Slang

1. a miserable, depressed (and depressing) person, usu. thus singled out in an institution, such as prison or the army.

1943		Billboard 11 Sept. 8/2: Nor should this be taken as the sour- grape griping of a sad sack who was shut out of news sources.
1945		in T. Shibutani Derelicts of Company K (1978) 214: Some of the expressions turned out to be quite humorous and were adopted throughout the company. ‘Sad sack’ became kanashii fukuro, literally a ‘melancholy bag’.
1949		(con. 1943–5) A. Murphy To Hell and Back (1950) 57: I got great news for you sad sacks [...] We attack.
1957		Lieber & Stoller ‘Jailhouse Rock’ 🎵 The sad sack was a sittin’ on a block of stone / Way over in the corner weepin’ all alone.
1968		(con. WWII) W. Stevens Gunner 63: They don’t want any more of these sad sacks.
1978		R. Price Ladies’ Man (1985) 61: Ten sad sacks sitting around the bar leaning cheeks on fists in front of their drinks.
1983		R. Price Breaks 320: Some Sad Sack with coat hanger posture and a big long nose.
1985		N. Pileggi Wiseguy (2001) 123: She was such a sad sack.
1991		W.T. Vollmann Whores for Gloria 75: I used to be a sad sack myself until I met my wife Gloria.
2000		Indep. on Sun. Culture 5 Mar. 14: There’s no need to be a sad sack.
2011		D.R. Pollock Devil All the Time 186: [T]he sad sack would think twice about ever doing himself in.

2. attrib. use of sense 1; cite 1944 suggests moral failing.

1944		D. Burley N.Y. Amsterdam News 8 Apr. 8B: [of an adulterer] This ‘Sad-Sack’ lover hits the turf [...] just in time to catch all the Heavy Hens pulling a creep from the bustle of their daily hustle.
1959		W. Brown Teen-Age Mafia 123: [...] moping around like some sad-sack square.
1998		K. Anderson Night Dogs 208: ‘Sorry-ass sad sack-looking motherfuckers too’.
1999		Guardian Weekend 14 Aug. 3: What a sad-sack selection of septuagenarians.
2002		D.H. Sterry Chicken (2003) 54: The ballerinas stare at me dolefully and plead with big sad-sack plastic eyes.
2004		Simon & Lehane ‘Dead Soldiers’ Wire ser. 3 ep. 3 [TV script] Was he as full of shit asd every other sad sack motherfucker wearing the badge of Baltimore police.
2008		R. Price Lush Life 253: Steele stared at him with his sad-sack eyes .
2015		P. Doyle (con. 1969-1973) Big Whatever 14: My sad-sack partner Johnny.
2016		Rolling Stone 14 Oct. 🌐 Pious Mike Pence, Trump's sad-sack running mate.