pin down

pin down
pin somebody/something ↔ down phrasal verb (see also pin)
1. to make someone give clear details or make a definite decision about something
    pin somebody/something ↔ down to
    Did you manage to pin him down to a definite date?
    He’s impossible to pin down.
2. to understand something clearly or be able to describe it exactly
    hard/difficult to pin down
    The flavour was hard to pin down.
3. if soldiers etc involved in fighting are pinned down, they cannot move from their position because someone is shooting at them:
    The rebels have been pinned down in a camp to the south of the river.