Why would I want anyone to do that? What would it mean if they did? What does my ass, Sally, or the two together, have to do with a state of disbelief or surprise?
I heard this exclamation uttered again yesterday, I hope for the very last time; and yet I can’t stop thinking about it. I must get to the bottom of this annoyingly inscrutable phrase! What does it mean? What…does…it…mean?
First you have to find out if it’s Mustang Sally or not. This is critically important. And if it is, find out if she ‘goes round the roses’. Get back to me.
My boss, who is from the vicinity of Kansas City, MO; uses as an expression of surprise: “Well butter my buns and call me a biscuit!” :eek: I have no clue how to respond to that…
One of my sister’s best friends used to alternate between:
“Well cut my hair off and call me baldy!”
and
“Cut my legs off and call me shorty!”
I grew up in southwestern Indiana, so I’m not sure if it’s a regional thing, a national thing or just something really weird that this particular person or her family did. It was funny as hell, though, because sometimes she’d get them mixed up and yell “Cut my hair off and call me shorty!”
I have always preffered “Well tie me up and dip me in malto meal!”
“Well, cut off me leg and call me Peggy.” Perhaps Sally was a close friend, and a slap isn’t quite as devestating as an amputation.
Well suck my nipples and call me momma!
Well, knock me down and call me scuttlebutt.
The line you heard was likely inspired (perhaps very indirectly) by Jon Lovitz. In the episode of Friends titled “The One with the Stoned Guy”, Lovitz was “one toke over the line” when he exclaimed: “Slap my ass and call me Judy!” See the Terms of Endearment section on this page.
There’s a hot sauce by that name.
Well, tuck me in and call me for breakfast!
Another one I’ve heard is “Well tie me to an Anthill and fill my ear with jam!”
That I get. It’s a kind of torture. Conceptually, it makes perfect sense. I might easily be confounded and/or surprised to find myself in such a predicament.
But imagine…
<Loopy, just standing there, minding his own business>
<Up runs some irritating person>
SMACK!
Loopy: “Hey! What the…”
Irritant: “Sally! Hah-hah!”
Loopy: 
Well, shellac my ass and call me shiny!
Speaking as the ambassador from the American South, ‘Sally,’ or ‘Sal’ are two traditional names for mules. Does that in any way help?
Mr Garrison once said that line on South Park.
Well, dip me in chocolate and throw me to the nymphos!
My personal version of this (learned from a friend’s dad - thanks, Al!) is
“Well dip me in shit and roll me in cracker crumbs!”
Makes me giggle every time.
I sometimes say, “Well, paint me blue and call me a Smurf.”
As to the OP, I have no fucking idea why people say these things. But they’re amusing nonetheless.
Why do I think that the next line is “No, really. Please. I mean it.” 
Grammar hijack: When you end a question with a quoted sentence that is not a question, how do you properly punctuate? I know that normally the punctuation does inside the quotation marks. But here, you actually need two punctuaters - one for the quote, one for the sentence (question).
well, smack me with a bag of rabbits.