Why is "my giddy aunt" an exclamation of surprise

The mental picture that forms in my mind is of fancy English ballrooms, a fainting lady and smelling salts.

So someone shouting "My giddy aunt" would rather be an expression of concern.

Does anyone have an explanation for this phrase?

Google Books and the Phrase Finder are not proving helpful and just term it "An exclamation of surprise."

Can one of the English scholars out there provide the colourful backstory to this archaic phrase?

The dictionary of slang you link to tells us "my giddy aunt!" comes from 1919 (from the pen of W. N. P. Barbellion) and that it's an elaboration of "my aunt!".

The same dictionary also gives some variations of my aunt!: my sainted aunt!, a mild exclamation since ca. 1920; my hat!; my stars!.

This may be unrelated, but it also notes that aunt was a procuress, concubine or prostitute (C.17 - ca. 1830) and cites Shakespeare.

[Oh,] my giddy aunt! is used as a playful euphemism to avoid blasphemy and is thought to have derived from (if not quoted from) the farcical comedy Charley's Aunt by Brandon Thomas that ran for 1,466 performances on its first production starting December 1892.

Yet more variations include my sainted aunts!, my sacred aunts! (cf. saints alive!).

Dipping into Google Books, the earliest giddy aunt is in 1837's The Monthly Review:

It may be enough to state, that a country squire's daughter, noble by the mother's side, is sent to the metrolis, and under the protection of a fashionable but giddy aunt, who happens to be a duchess, and a widow, proud and poor, is appointed to entrap a coronet.

There are several more actual aunts who are giddy, but the first exclamation "my giddy aunt!" is in 1894's McClure's magazine, Volume 2 by S.S. McClure, 1894:

"The-old-devil! From the mouths of babes and suckings! Kuala! Oh, my giddy aunt!" He went off into a peal of merriment—and the girl in the coat embroidered with silver flowers and butterflies surveyed him with amazement.

The next references are from 1908, 1910, 1913, and the aforementioned 1919.

Green's Dictionary of Slang has my aunt!, my aunt Eliza, my Aunt Jane, my aunt Nellie, my fat aunt, my giddy aunt, my great aunt, my hairy aunt, my holy aunt, my precious aunt and my sainted aunt meaning the same thing, a mild exclamation; the earliest citation is from 1678.

From the expression my hairy aunt and a nearby heading ask my aunt, I am inclined to suspect that aunt in all these expressions is a euphemism for arse.