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(Well,) I'll be darned!

old-fashioned An expression of surprise or astonishment, especially regarding some recent revelation. A: "You know, if you run your washing machine at night you can save a lot of money on your electric bill." B: "I'll be darned, I didn't know that!" Well, I'll be darned! Who knew so many people would show up to see our concert? Wait, Lisa's your sister? Well, I'll be darned—all these years, I had no idea!
Farlex Dictionary of Idioms. © 2022 Farlex, Inc, all rights reserved.

I’ll be ˈdarned!

(spoken, especially American English) used to show that you are surprised about something: Well, I’ll be darned! Isn’t that Lisa over there?
Farlex Partner Idioms Dictionary © Farlex 2017
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References in periodicals archive ?
So frankly it's about time people stopped being so darned pessimistic and looked at what we're good at in this country."