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.

Common Names:

Name	Pronounced	Usage
Ormazd	OR-mazd	Persian Mythology
Izolda	ee-ZAWL-dah (Polish)	Georgian, Polish
Rain (2)	-	Estonian
Elpidios	-	Late Greek
Sacheverell	sə-SHEV-ər-əl	English (Rare)
Gherardo	ge-RAHR-do	Italian (Archaic)