Definition:
: a person connected to an activity, an organization, a movement, etc.
About the Word:
"You're always so persistent about things. You're such a nudnik. Why do you ask me to eat shrimp? What the hell is so big about shrimp?" — Thomas J. Cottle, When the Music Stopped: Discovering My Mother, 2004
Example:
There are many different kinds of -niks, a handy word ending which comes from the Yiddish suffix of the same spelling, and denotes a person who is connected to a group or a cause. Among the more common words that utilize this are beatnik, neatnik, peacenik, nudnick, and the ever popular no-goodnik. Russian also has this suffix, and English has borrowed -nik words from Russian too, including refusenik and Raskolnik (a dissenter from the Russian Orthodox Church).
