pin down

verb

1. To attach or secure with pins.

   “Pin down the tablecloth, so it doesn't blow away in the breeze.”

   “There are approximately 1,000 places on British Railways where wagon brakes must be pinned down before trains proceed over severe falling gradients; the trains have to be brought to a stand for the brakes to be pinned down, and subsequently have again to be stopped for the brakes to be released.”

2. To identify something (e.g. a decision or plan) clearly or specifically.

   “Something is wrong, but I can't pin it down.”

3. To hold the opponent back-first so both his or her shoulders touch the mat or ground.
4. To corner somebody in order to get a firm answer.

   “Let's try to pin him down on a price.”