nail something down

1. fasten something securely with nails.

〈物〉をくぎで留める

2. identify something precisely:

〈原因など〉を明らかにする, 突き止める

e.g. something seems unexpected—I can't nail it down, but it makes me uneasy.

3. secure something, especially an agreement:

｟主に米｠ A〈協定など〉を確定する, 最終決定する.

e.g. the company has finally nailed down the agreement with its distributors.