Squirrel Trying to Crack a Hazelnut

a tough/hard nut to crack
깨기 힘든 너트
The expression refers to a problem that is difficult to solve or a person who is difficult to deal with.
이 표현은 해결하기 어려운 문제나 다루기 힘든 사람을 뜻합니다. 🙂😉😆