tough nut to crack

Noun

 * 1)  A problem that is challenging to solve.
 * 2)  A situation, individual, group, etc. that is difficult to deal with.
 * 3)  A place, opportunity, etc. to which it is difficult to gain entry.
 * 4)  An amount that is difficult to finance.

Usage notes

 * Tough/er/est nut to crack is by far the most common form in COCA (US), about equal to hard/er/est nut to crack at BNC (UK).

Translations

 * Finnish: kova pähkinä purtavaksi
 * German: harte Nuss
 * Italian: gatta da pelare
 * Russian:
 * Spanish: hueso duro de roer
 * Swedish: svår nöt att knäcka