nail down

nail down  {v. phr.},  {informal}
To make certain; make sure; settle.
Joe had a hard time selling his car, but he finally nailed the sale down when he got his friend Sam to give him $300.
The New York Yankees nailed down the American League Championship when they beat the Red Sox 3 to 0 on September 15.
Categories: informal verb
Source: A Dictionary of American Idioms

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