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?