WordNet (r) 3.0 (2006):

stumblebum
    n 1: a second-rate prize fighter [syn: stumblebum, palooka]
    2: an awkward stupid person [syn: lout, clod, stumblebum,
       goon, oaf, lubber, lummox, lump, gawk]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "stumblebum":
   blunderbuss, blunderer, bungler, butterfingers, clumsy, dub,
   duffer, foozler, foul-up, fumble-fist, goof, goofball, goofer,
   hick, incompetent, lummox, lump, muff, muffer, rube, slob,
   stumblebunny