This phrase draws on the assumption, a staple of detective fiction, that the person found with a recently fired gun must be the guilty party. The use of the phrase in the late 20th century was particularly associated with the Watergate scandal in the early 1970s involving the US President Richard Nixon. When one of the Watergate tapes revealed Nixon's wish to limit the FBI's role in the investigation, Barber B. Conable famously commented : I guess we have found the smoking pistol, haven't we?
1998 - New Scientist - This genetic smoking gun is evidence of a migration out of Asia that is hard to refute.
RELATED IDIOMS :
a big gun = a big cheese = a big fish = a big noise = a big shot = a big wheel
an important and influential person – informal
These are mainly self-explanatory with the exception of cheese itself which is of doubtful origin but may be from Persian and Urdu meaning thing. As a phrase, big cheese seems to have originated in early20th-century US slang, as did big noise. Big wheel in this metaphorical sense (as opposed to the fairground ride known as a Ferris wheel) and big shot are similarly US in origin (mid 20th century). Big fish may have connotations either of something it is desirable for you to catch or of the metaphorical expression a big fish in a small pond.
blow great guns
be very windy – informal
go down with guns firing = go down with all guns firing
fail or be beaten, but continue to offer resistance until the end.
go great guns
perform forcefully, vigorously or successfully – informal
1913 - Field - A moment later Louvois shot out, passed Sanquhar and Fairy King and
going great guns and beat the favourite by a head.
jump the gun
act before the proper or appropriate time – informal
IN athletics, a competitor who jumps the gun sets off before the starting pistol has been fired. The expression appears in the early 20th century as beat the gun.
stick to your guns
refuse to compromise or change, despite criticism - informal
The image here is of a soldier maintaining his position under enemy fire.
1998 - New Scientist - Researchers have bravely stuck to their guns as they went about seeking public funds.
a (or the) most important person
under the gun
under great pressure – North American informal
with guns blazing = with all guns blazing
with great determination and energy, often without thought for the consequences - informal