tough nut to crack

Noun

idiomatic broadly

Japanese Meaning

解決が難しい問題 / 対処が困難な状況、人、集団など

Fixing the legacy billing system proved to be a tough nut to crack for the development team.

既存の請求システムを修正することは、開発チームにとって解決が難しい課題だと分かった。

Sense（1）

(idiomatic) A problem that is challenging to solve.

Sense（2）

(idiomatic, by extension) A situation, person, group, etc. which is difficult to deal with.

Sense（3）

(idiomatic) A place, opportunity, etc. to which it is difficult to gain entry.

Sense（4）

An amount that is difficult to finance.

tough nuts to crack

（ plural ）