Nudniks are usually to be found in the USA. Well, that's the place where they'll be called nudniks, anyway, (you say it NUDDnick) but actually you'll find them pretty much everywhere.
A nudnik is an annoying person. He (or she, or it (does that cover all the bases?)) might be a persistent nag, a bore, a pest or even a jerk.
You know that guest who spends the whole dinner party telling everyone how the food should have been cooked? The neighbour who complains constantly about the sound of lawn mowers? The colleague who prides himself on what he calls winding people up? The man at the party who's an expert in something, but his voice is so boring your eyes glazed over before you found out what it was? All nudniks.
Oh, the relief of having a word for one of those at last!
Spot the Frippet (and then run): a nudnik. This word comes to English through Yiddish, and before that from Polish and Ukrainian. The Polish nuda means boredom, and the -nik means connected with or characterised by.