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.
See also: egg, tough

tough egg to crack

and 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.
See also: crack, egg, tough
References in periodicals archive ?
The fact my grandmother probably understands more about blu-ray players than me means I may be a tough egg to crack.
However the 26-year-old light-heavy prospect admitted that 36-year-old Okoth was a tough egg to crack.