nail down

verb

1. to fasten down with or as if with nails

2. informal,  to extort a definite promise or consent from

   I nailed him down on the deadline

3. informal,  to settle in a definite way

   they nailed down the agreement

Idioms and Phrases

Establish conclusively, as in The reporter nailed down the story by checking all the facts. This metaphoric expression alludes to fixing or fastening something down with nails. [c. 1600]