Definition: A nudnik is a pest. When a child asks, 100 times an hour "are we there yet?" the child is being a nudnik. But nudnik is usually used for adults who constantly bother you with questions or demands. Do not confuse nudnik with patzer, which is someone who gives unwanted advice. Also do not confuse nudnik with noodzhe (or noodge) which is someone who tries to give you a little push, but might do it only once. Nor is a nudnik a kvetch - that's someone who complains all the time.
Pronunciation: Rhymes with "would Nick?"
Origin: Nudnik comes from Yiddish
Why use it? Because, as Leo Rosten said: "Yiddish is like every other language, only more so". Nudnik is just a great word for a particular kind of nagging pest.
Examples: There is a joke (of course!) A guy is traveling on a train with no drinks available. The woman across the aisle says, over and over and over and over, "Oy! Am I thirsty!" When the train comes to a stop, the guy dashes out of train, races to buy some water, rushes back in and gives it to her.
"Thank you" she says. "You're welcome". And he sits down hoping for some quiet but ....
"Oy! Was I thirsty!"
Sources: The Joys of Yiddish by Leo Rosten
Your turn: Should I define more Yiddish words?