動詞 pin down:

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

1. It was difficult to pin down exactly what it was in her that pleased him so much.
   很難確切說清她身上有什么東西使他這樣動心。
2. She's a difficult person to pin down.
   她這人行事、說話沒個準兒。