The police have been tightlipped about the details of the accident.

tightlipped 是由形容词 tight（紧的）和名词 lip（嘴唇）加上后缀 -ed 组成的复合形容词。字面意思是"嘴唇紧闭的"，引申为"不愿透露信息的"或"守口如瓶的"。