Q: Why didn’t the zombie go to work?
A: Because he felt rotten!
Explanation: When you are sick, it is very important that you don’t go to work or school, especially now with COVID still making people sick. If you feel really sick you might say that you feel rotten; to feel rotten is an idiom meaning to feel sick.
A zombie is a dead body (corpse) that has come back to life. (By the way, zombies are fictional, mythological, not real.) Once a body stops living, the body begins to decompose, to rot. So, a zombie’s body is in the process of rotting.
This joke is funny because it plays with the idiom to feel rotten and the verb to rot.
Speaking of zombies, this seems like a good time to revisit Michael Jackson’s Thriller-