When do you use ';nail down';?

When do you use ';nail down';?


and also Does ';nail down'; have a connection with ';nail';?When do you use ';nail down';?
It's slang for ';let's finalize this.'; For example, ';let's nail down the date for the party.'; I suppose that if you literally nail something down (that is, use a nail and hammer it into something) then it is fairly permanent.