Jump the gun
Look up jump the gun in Wiktionary, the free dictionary.
The idiom Jump the gun[1] has its roots in an athlete starting a race before the starting pistol is fired. It refers to someone or to an act when it is started earlier than expected, or when something is spoken without required thinking. In finance, it refers to a scenario where various financial instruments are traded based on some knowledge that is not yet public.
References
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