explode a bombshell
explode a bombshell or drop a bombshell {v. phr.}, {informal}
To say something startling; suggest or show something astonishing or shocking.
The police exploded a bombshell when they arrested the kindly old banker for stealing money from the bank.
The principal exploded a bombshell by cancelling the dance as a penalty.
Political leaders exploded a bombshell when they picked the young lawyer to run for mayor.
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Source: A Dictionary of American Idioms