noun
the hardest part; the most trying stage
Refers to the most difficult or critical point in a long effort, often used for the final stretch of a task or journey. Originally from the steepest section of a mountain path.
このプロジェクトは胸突き八丁に差し掛かっている。
This project is reaching its most difficult stage.
登山は胸突き八丁
Once you get past the steepest part of the climb, the rest is easy.