drop a bombshell
Make an unexpected or shattering announcement. For example, Bill dropped a bombshell when he said he was quitting. This expression, which alludes to the destruction caused by a falling bomb, dates from World War I.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.