To pin down somebody is to get a clear answer out of them. Hard to pin down, pin down a date.
It implies the other person has been vague on purpose. It is also used of a fact or a feeling that resists description, which is hard to pin down in a different way.
1phrasepin downTo get a clear answer out of somebody.
“Nobody could pin down when the work would actually start.”
pin down is a phrase.
Practised in Phrasal Verbs