The Collaborative International Dictionary of English v.0.48:

-nik \-nik\ (n[i^]k) suff. [Yiddish, fr. Slavik suffix -nik
   indicating a person.]
   A suffix attached to other words indicating a person with
   certain characteristics or associated with a certain group or
   behavior; as, beatnik, kibbutznik; it is sometimes used
   derogatorily; as in no-goodnik, peacenik.
   [PJC]