Meaning of “Muzui(むずい)”

“Muzui(むずい)” is an abstraction for “Muzukashii(むずかしい)” which means difficult. This is a slang that has been used for a long time and is quite a common expression.
Also, “Muzukunai(むずくない)?” is an abstraction for “Muzukasikunai(むずかしくない)?” which means “Isn’t it difficult?”.

Example Conversation

A: Can you solve this problem?
B: Oh, muzukunai(むずくない)?
C: Yes, super muzui(むずい)…

A: この問題とける？
B: え、これかなりむずくない？
C: 相当むずいね…。