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]