The Collaborative International Dictionary of English v.0.48:

nudnik \nud"nik\ (n[oo^]d"n[i^]k) n. [Yiddish, fr. Russ. nudnyi
   tiresome.]
   a pestiferous boring and dull person.
   [PJC]

WordNet (r) 3.0 (2006):

nudnik
    n 1: (Yiddish) someone who is a boring pest [syn: nudnik,
         nudnick]