smoking gun
noun [ C usually singular ] uk/ˌsməʊ.kɪŋ ˈɡʌn/ us/ˌsmoʊ.kɪŋ ˈɡʌn/
确凿的罪证
information that proves who committed a crime
The tape recordings provided prosecutors with the smoking gun they needed to prove he'd been involved in the conspiracy. 录音带为检察官们提供了他们所需要的确凿证据，证明他参与了这一阴谋。