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bolt from the blue, a

bolt from the blue, a idiom

Also, a bolt out of the blue. A sudden, unexpected event. For example, Bill's dropping his life insurance was a bolt from the blue for his wife. This metaphoric term alludes to totally unforeseen lightning or thunder from a cloudless (blue) sky. [First half of 1800s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.