Nudnick is Yiddish. The -nik ending is a dead giveaway.

Per Rosten: a nudnik is not just a nuisance; to merit the status of nudnik, a nuisance must be a persistent, talkative, obnoxious and indefatigable nag. As in a mother saying to a child, "Stop bothering me. Don't be a nudnik!" So yes, there is a component of endearment.

Illustrative:
1. What is phudnik? A nudnick with a Ph.D.

2. Mr. Polanski complained to his doctor, "Something terrible has happened to me. I try to stop it, but I can't ... Morning, noon, and night -- I keep talking to myself!" "Now, now," the doctor crooned, "that isn't such a bad habit. Why, thousands of people do it." "But Doctor," protested Polanski, "do you know what a nudnik I am?!"