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.

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.

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]

A difficult problem; a hard person to deal with. This early analogy, also put as a hard nut to crack, was first drawn in the early eighteenth century. Benjamin Franklin used it in a letter in 1745: “Fortified towns are hard nuts to crack; and your teeth have not been accustomed to it.” A similar term from a somewhat later era is tough customer, likewise meaning a person difficult to deal with. Dickens used it in Barnaby Rudge (1841): “Rather a tough customer in argument, Joe, if anybody was to try and tackle him.”