I'll be darned!

(well,) I'll be darned!

old-fashioned An expression of surprise or astonishment, especially regarding some recent revelation. Bob: "You know, if you run your washing machine at night you can save a lot of money on your electric bill." John: "I'll be darned, I never knew that before!" Well, I'll be darned! Who knew so many people would show up to see our concert?

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?