What does derk mean in yiddish?

"Derk" is not a word in Yiddish. It's possible you're thinking of a different word, or it could be a misspelling.

Could you provide more context?

For example:

* What was the word used in? A sentence? A song?

* How was it spelled?

* What was the meaning you thought it had?

With more information, I can help you figure out the correct word and its meaning.