a hard nut (to crack)

a hard nut (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 hard nut to crack! Figuring out the best way to modernize our product without alienating existing customers is definitely a hard nut to crack. The spy we captured is a hard nut; he hasn't said a thing since we began the interrogation.
See also: hard, nut

(a) hard nut to crack

 and (a) tough nut to crack
Fig. difficult person or problem to deal with. This problem is getting me down. It's a hard nut to crack. Tom sure is a hard nut to crack. I can't figure him out.
See also: crack, hard, nut

hard nut to crack

Also, tough nut to crack. A difficult problem; also, an individual who is difficult to deal with. For example, This assignment is a hard nut to crack, or It won't be easy getting her approval; she's a tough nut to crack. This metaphoric expression alludes to hard-shelled nuts like walnuts. [Early 1700s]
See also: crack, hard, nut