pin down

pin down  {v.}

1a. To keep (someone) from moving; make stay in a place or position; trap.

Mr. Jones' leg was pinned down under the car after the accident.

The soldier was pinned down in the hole because rifle bullets were flying over his head.

1b. To keep (someone) from changing what (he) says or means; make (someone) admit the truth; make (someone) agree to something.

Mary didn't like the book but I couldn't pin her down to say what she didn't like about it.

I tried to pin Bob down to fix my bicycle tomorrow, but he wouldn't say that he could.

2. To tell clearly and exactly; explain so that there is no doubt.