The Collaborative International Dictionary of English v.0.48:

Nebbish \Neb"bish\ (m[e^]b"[i^]sh) n. [Yiddish, nebekh poor,
   unfortunate.]
   A person who is ineffectual, timid, and often luckless; a
   person of no significance.
   [PJC]

WordNet (r) 3.0 (2006):

nebbish
    n 1: (Yiddish) a timid unfortunate simpleton [syn: nebbish,
         nebbech]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "nebbish":
   Milquetoast, a nobody, a nothing, baby, big baby, chicken, cipher,
   common man, crybaby, doormat, dud, dull tool, dummy, figurehead,
   gutless wonder, hollow man, insignificancy, invertebrate,
   jackstraw, jellyfish, lay figure, lightweight, little fellow,
   little guy, man of straw, mediocrity, meek soul, milksop,
   mollycoddle, namby-pamby, nobody, nobody one knows, nonentity,
   nothing, nullity, obscurity, pansy, pantywaist, pip-squeak, punk,
   puppet, pushover, runt, sad sack, scrub, shrimp, sissy, small fry,
   small potato, small potatoes, softling, softy, sop, squirt, squit,
   thing of naught, trifle, weak sister, weakling, whiffet,
   whippersnapper, zero