Learner's definition of BOMBSHELL
1 : something that is very surprising or shocking — usually singular
The book was a political bombshell.
The news of his departure was a bombshell.
She dropped a bombshell [=she surprised everyone] when she said she wouldn't run for reelection.
2 informal : a very attractive woman — usually used in the phrase blond/blonde bombshell to refer to an attractive woman with blonde hair