Real Dictionary

nail down
phrasal verb: If you nail down an agreement, you manage to reach a firm agreement with a definite result.
phrasal verb: If you nail down something unknown or uncertain, you find out exactly what it is.
phrasal verb: If you nail something down, you fix it firmly onto something.

nail down in American English
to fasten tightly with nails
to make final; settle once and for all

nail down in British English
to fasten down with or as if with nails