smoking gun
noun /ˌsməʊkɪŋ ˈɡʌn/
/ˌsməʊkɪŋ ˈɡʌn/
[singular] (informal)- something that seems to prove that somebody has done something wrong or illegal
- This memo could be the smoking gun that investigators have been looking for.
- There's no smoking gun that they have found to date.