As you know, somewhere in The adventures of Huckleberry Finn, Jim expresses his certainty that he's noticed that a noise came from the garden of Miss Watson by saying (my emphasis)
"Say, who is you? Whar is you? Dog my cats ef I didn' hear sumf'n..."
I wonder if: a) the expression in boldface was a common one in those days (at least, in the area where the story develops), b) it is totally obsolete these days, and c) you know expressions that are equivalent to it and not at all uncommon to listen nowadays.
Why do I believe that it may have been an actual expression from those times? To begin with, there is the disclaimer by M. Twain that appears at the beginning of the book:
IN this book a number of dialects are used, to wit: the Missouri negro dialect; the extremest form of the backwoods Southwestern dialect; the ordinary "Pike County" dialect; and four modified varieties of this last. The shadings have not been done in a haphazard fashion, or by guesswork; but painstakingly, and with the trustworthy guidance and support of personal familiarity with these several forms of speech.
I make this explanation for the reason that without it many readers would suppose that all these characters were trying to talk alike and not succeeding.