Definition of nail down phrasal verb from the Oxford Advanced Learner's Dictionary

 

nail down

 phrasal verb
phrasal verb
 
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Phrasal Verbs
  1. 1to fasten something down with a nail or nails
  2. 2to reach an agreement or a decision, usually after a lot of discussion All the parties seem anxious to nail down a ceasefire.

nail somebodydown (to something)

 
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to force somebody to give you a definite promise or tell you exactly what they intend to do synonym pin down She says she'll come, but I can't nail her down to a specific time.
See the Oxford Advanced American Dictionary entry: nail down