hardest nut to crack

Noun

hard nut to crack (plural hard nuts to crack)

1. (idiomatic) A problem that is challenging to solve.
   • He saw the dilemma. . . . It was a hard nut to crack. He could see no way out of it.1893, Grant Allen, chapter 11, in Michael's Crag
2. (broadly, idiomatic) A situation, person, group, etc. which is difficult to overcome or deal with.
   • The coast lad found the veteran Mike McTigue a hard nut to crack and judging from the look on the Californian's face when the final bell sounded, he was mighty happy that the fight was over.1928 June 8, “Mike McTigue Gives Emanuel a Real Fight”, in Milwaukee Journal, retrieved 22 Sep 2011, page 2
3. (idiomatic) A place, opportunity, etc. to which it is difficult to gain entry.
   • The next day Archie, with Andrew Macpherson and Cluny Campbell, made their way through the woods until within sight of the castle. . . .
     "It would be a hard nut to crack, Sir Archie," his lieutenant said. "Unless by famine, the place could scarce be taken."1885, G. A. Henty, chapter 7, in In Freedom's Cause
4. (idiomatic) An amount that is difficult to finance.
5. Used other than figuratively or idiomatically: see hard, nut, crack.

   Synonyms:

   difficult nut to crack, tough nut to crack