解かい決けつの糸いと口ぐち

expression, noun

key to a solution; first step toward a solution; thread to unravel a mystery

Refers to a clue, hint, or initial breakthrough that leads to solving a problem or mystery. Often used when a difficult situation begins to show a way forward.

• 長なが年ねんの研けん究きゅうの末すえ、ついに解かい決けつの糸いと口ぐちを見みつけた。

  After years of research, I finally found a key to the solution.

• このデータが事じ件けんの解かい決けつの糸いと口ぐちになるかもしれない。

  This data might be the thread to unravel the case.