I was watching a former Watergate prosecutor interview on CNN and heard the following passage:
one of the things that the House Judiciary Committee, the House Intelligence Committee absolutely has to do is really button down whatever versions of this memo exist, whatever notes exist, they have to get all of the witnesses to the conversation in and make sure that they actually button down that what is represented there is the conversation
You may hear this passage here.
I looked button down in dictionaries but it doesn't seem to be a firmly established idiom. I googled it and also without success. I wonder if this phrase is a real idiom or I just misheard.
My understanding is that it can mean to ascertain or to pin down, but I want to know for sure.