tough egg to crack
a tough egg (to crack)
A person, thing, situation, or problem that is particularly difficult to understand, solve, or deal with. I've been dating Jenny for over a year, and I still think she's a tough egg to crack! Figuring out the best way to modernize our product without alienating existing customers is definitely a tough egg to crack. The spy we captured is a tough egg; he hasn't said a thing since we began the interrogation.
tough egg to crackand tough nut to crack
n. a person or thing that is hard to figure out or hard to deal with. She won’t talk. She’s going to be a tough nut to crack! I wish Jill wasn’t such a tough nut to crack.