1. something very complicated to understand or to do; difficult person or problem to deal with

1. Figuring out how to get the elephant into Smith’s cab is a tough nut to crack.

2. This problem is getting me down. It’s a hard nut to crack.

3. Tom sure is a hard nut to crack. I can’t figure him out.

4. He's a tough nut, but I think I can get him to agree to the contract.