pin down

动词 pin down:

1. define clearly
   同义词：peg down, nail down, narrow down, narrow, specify
2. attach with or as if with a pin
   同义词：pin up

1. It was difficult to pin down exactly what it was in her that pleased him so much.
   很难确切说清她身上有什么东西使他这样动心。
2. She's a difficult person to pin down.
   她这人行事、说话没个准儿。