tough nut to crack, a

a tough nut to crack

1. A difficult task to complete. Getting an A in this class will be a tough nut to crack. You want me to find more money in the budget? Ha, there's a tough nut to crack!
2. A challenging or unreasonable person to understand or deal with. Our super-strict principal is a tough nut to crack, so I hope Kate can convince her to let us host this event. Because our boss usually keeps to himself, I have no idea what his interests are—he's a tough nut to crack.
See also: crack, nut, tough
Farlex Dictionary of Idioms.

(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
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

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
The American Heritage® Dictionary of Idioms by Christine Ammer.

a tough nut to crack

or

a hard nut to crack

1. If you say that a problem is a tough nut to crack or a hard nut to crack, you mean that it is difficult to solve. The really tough nut to crack will be to persuade the older staff that change is necessary. The American market is a very hard nut to crack because it is so vast. Note: You can also just refer to a difficult problem as a tough nut or a hard nut. The tough nut for this government is undoubtedly the economy.
2. If you say that someone is a tough nut to crack or a hard nut to crack, you mean that they are difficult to defeat. Harrington has taken 17.5 points from a possible 20 in international singles, making him a tough nut to crack. Three wins and three draws prove United are a hard nut to crack.
See also: crack, nut, tough
Collins COBUILD Idioms Dictionary, 3rd ed.

tough nut to crack

verb
See tough egg to crack
See also: crack, nut, tough
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions

tough nut to crack, a

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.”
See also: nut, tough
The Dictionary of Clichés by Christine Ammer
See also:
  • a tough nut to crack
  • tough nut to crack
  • be tough
  • be/get tough
  • hang tough
  • tough times
  • (as) tough as nails
  • tough as nails
  • follow
  • tough