smoking gun

noun
indisputable proof or evidence of a crime.

Origin:
1970–75

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
smoking gun
 
n
a piece of irrefutable incriminating evidence

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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American Heritage
Idioms & Phrases

smoking gun

Something that serves as indisputable evidence or proof, especially of a crime. For example, There is no smoking gun in the Oval Office; the President had no role in tampering with the evidence. This expression alludes to the smoke coming from a recently discharged firearm, a normal occurrence until the invention of smokeless powder. [Mid-1900s]

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
The guess here is that minus any forthcoming smoking gun evidence, he will get
  in.
It even has action shots of code that it says are a smoking gun.
She intends this to be something of a bombshell: the smoking gun in the form of
  a spray nozzle, as it were.
In lieu of a smoking gun, a more complex picture of autism's environmental
  causes is now emerging.
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