WordNet (r) 3.0 (2006):

sad sack
    n 1: someone who makes mistakes because of incompetence [syn:
         bungler, blunderer, fumbler, bumbler, stumbler,
         sad sack, botcher, butcher, fuckup]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "sad sack":
   Bowery bum, Milquetoast, baby, beachcomber, beggar,
   beggarly fellow, big baby, blighter, budmash, bum, bummer, caitiff,
   chicken, crybaby, derelict, devil, doormat, drifter, drunkard,
   dull tool, good-for-naught, good-for-nothing, gutless wonder,
   hard-luck guy, hardcase, hobo, human wreck, invertebrate,
   jellyfish, lightweight, loser, lowlife, mauvais sujet, mean wretch,
   meek soul, milksop, mollycoddle, mucker, namby-pamby, nebbish,
   no-good, nonentity, pansy, pantywaist, pauvre diable, pilgarlic,
   poor creature, poor devil, poor unfortunate, pushover, sad case,
   schlemiel, schlimazel, sissy, skid-row bum, softling, softy, sop,
   stiff, sundowner, sure loser, swagman, tramp, truant, unfortunate,
   vag, vagabond, vagrant, vaurien, victim, wastrel, weak sister,
   weakling, worthless fellow, wretch