I (do) declare! old-fashioned expression informal
Used to express surprise.
A: Congratulations! You have passed the exam. B: Well, I declare!
Peter: Did you know that she gained promotion? Linda: I declare!
To be surprised by some sudden occurrence
To greatly surprise, shock, or confuse someone
Meaning absolutely certain or without a doubt
I am as sure as God made little green apples that Peter's the one for you, you two make a beautiful couple.