1. A stubborn, cynical person, especially an employee or colleague who resists efforts to improve. "Nut" is English slang for "head".
2. A tough problem that is hard to solve is a "hard nut to crack".
1. Jim tried everything to improve the audit team's morale, but the hard nuts just labelled him "lame."
2. Until the US economy recovers, unemployment will be a hard nut to crack.