darned

(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?
Farlex Dictionary of Idioms.

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
See also:
  • (Well,) I'll be darned!
  • I'll be darned!
  • blowed
  • I'll be blowed
  • (Well,) I'll be blowed!
  • I'll be hanged
  • I'll be!
  • (well,) I'll be hanged!
  • (well,) I'll be!
  • (Well,) I'll be durned!
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."
Mandy: We must start looking on bright side; ECONOMY