I have no idea what 'Wallop my Withers' mean!

[Molecular Kogwheel 37]

Well, while watching the show, I realized that Applejack, as a farmer, tends to speak with Southern American slangs and idioms, like, 'Y'all' and 'She's a few apples short of a bushel'. That's cool. I also realise some of those slangs and idioms and all have horse puns in it, like 'goody FOUR-shoes' and 'horse-drawn, horse-drawn carriages'. That's cool too. But, no matter how much I research, I possibly cannot find what is the origin of the phrase that Applejack says, 'Well,Wallop my Withers, Spike!' which is from Season 1 Episode 3 The Ticket Master! In fact, I know wallop means to hit or strike, and withers is the back of an animal between their shoulder blades, but I never heard the phrase, 'Well, hit my back, pardner'. Can somepony, anypony, please tell me what is the original phrase of 'Wallop my withers'?

[RrVPfX9cPtw59FpC 1,304]

I suppose it could be something like, "Slap me silly", but that's a really uncommon phrase. It's possible that the writers were just making a random saying, but I'm not really sure.

[Scootalove 10,689]

It's probably just a old Southern saying that the writers found. Applejack probably just says random sayings, that probably have no meaning.

[I used to be a stranger 7,994]

It was probably made up for the show, banking on the appeal of using alliterations. Wallop being a word that's fallen out of common parlance and thus being somewhat 'antiquated' could be excused for thinking it's a 'Southern' word, and pronounced with the accent, makes it sound like a common expression. Thus, the invention of a new phrase.