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.
Best Answer
"Dog my cats" is a minced oath, reflecting oaths of the type "God damn my eyes", or "Damn my soul". It was moderately common in the 19th century (you can generally trust Twain); just Google Books the phrase for that period.
It's not entirely obsolete, but in contemporary speech it's used pretty much jocularly, with quotes around it, like "Well hesh my mouf" or "Jeez Louise" or "Holy Maloney".
We don't worry so much about vulgar language as Twain's readers did, so contemporary equivalents would be less reticent and less colorful. If you need something generic, "Heavens" or "For heavens' sake" have been pretty much standard since Jacobean theatres and publishers started using it to replace the terms forbidden by the 1606 Act to Restrain Abuses of Players.