Exciting news! With our new Ad-Free Premium Subscription you can enjoy a distraction-free browsing experience while supporting our site's growth. Without ads, you have less distractions and enjoy faster page load times. Upgrade is optional. Find out more here, and enjoy ad-free learning with us!
From Wikipedia:
The term "smoking gun" was originally, and is still primarily, a reference to an object or fact that serves as conclusive evidence of a crime or similar act. Smoking gun - Wikipedia, the free encyclopedia
"Smoking gun" is used to describe any type of tell-tale evidence, or indisputable proof:
Little Jimmy swore to his Mom that he hadn't eaten the last piece of pie, but the crumbs and blueberry stains around his mouth were the smoking gun that led her to doubt him.