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Phrase(s): jump the gun
Fig. to start before the starting signal. (Originally used in sports contests that are started by firing a gun.) • We all had to start the race again because Jane jumped the gun. • When we took the test, Tom jumped the gun and started early.
McGrawhill's American Idioms And Phrasal Verbs
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Start doing something too soon, act too hastily. For example, The local weather bureau jumped the gun on predicting a storm; it didn't happen for another two days. This expression alludes to starting a race before the starter's gun has gone off, and supplants the earlier beat the pistol, which dates from about 1900. [Mid-1900s]
American Heritage Idioms