What does 젬병 mean?

 ANSWER: pancakes?

젬병 is Korean slang for "being terrible at" (형편없다) doing something. Here is an example sentence:

내가 과학에는 젬병이다

"I'm terrible at Science."

But 젬병 comes from 전병 (煎餠), which my Korean-English dictionary defines as Korean-style "pancakes." So that means the above Korean sentence can literally translate as follows: "I'm a pancake at Science."

Here is a photo of Korean 전병.