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Jiminy, by jiminy, jumpin' jiminy etc

—used as a mild oath often in the phrases by jiminy, jiminy crickets, jiminy Christmas

-Merriam Webster

In a more innocent age, and before the constant present-day usage of "fuck" as an expletive, there used to be some rather quaint expressions to express surprise, or shock. One of these was "Jiminy, or "by jiminy", or even "jumpin' jiminy".

Some people claim it comes from the Disney character Jiminy Cricket, but I can find references going back far beyond that, so I am guessing that the character was named after the exclamation (i.e. “Jumpin’ Jiminy”), rather than the reverse.

-The Cult of the Heavenly Twins (J. R. Harris, 1917)

There is some speculation that it derives from a Scandinavian expression, like “oofta”. There is also speculation that the initials i.e.JC is no coincidence, and is a way of saying "Jesus Christ!" as an expletive.

What is the origin of this expression? Is this an Americanism, or is it an exclamation coming from a Scandinavian (possibly Swedish) expression?

It comes from saying "Jiminy Christmas," a euphemism used in place of using the name "Jesus Christ" as a curse word. It's like saying "heck" instead of "hell" or "darn" instead of "damn" or "shut the front door" instead of "shut the f*ck up."

Here's what The Old Farmer's Almanac has to say about it:

“Jimmy Christmas” or “Jiminy Christmas” is a direct reference to Jesus
Christ and dates back to 1664, when it was first recorded as “Gemini,”
a twist on the Latin phrase Jesu domini. The name of the Walt Disney
character Jiminy Cricket was probably based on this phrase.

Now, a question I've always wondered is if the minced oath "criminitly," which is also a ephemism for "Jesus Christ," has anything to do with La Cosa Nostra.

And from Entymonline an interjection:

Jiminy (interj.) exclamation of surprise, 1803, colloquial form of
Gemini, a disguised oath, perhaps Jesu Domine "Jesus Lord." Extended
form jiminy cricket is attested from 1848, according to OED, and
suggests Jesus Christ (compare also Jiminy Christmas, 1890). It was in
popular use in print from c. 1901 and taken into the Pinocchio fairy
tale by Disney (1940) to answer to Italian Il Grillo Parlante "the
talking cricket."

And from the OED with early usage:

• Used as a mild oath or exclamation, esp. in Jiminy Christmas (see Christmas int.) and Jiminy cricket.


• 1803 G. Colman John Bull i. i. 6 Den. A customer... Mrs. B. Jemmeny!

and

c1816 Ld. Byron in R. G. Latham Dict. Eng. Lang. (1872) Crimini,
jimini! Did you ever hear such a nimminy pimminy Story as Leigh Hunt's
Rimini?

From Hugh Rawson, A Dictionary of Euphemisms and Other Doubletalk (1981):

Jiminy Cricket. The cute Walt Disney character notwithstanding, this is a euphemism for for "Jesus Christ," on a par with Judas Christopher, Judas Priest, cripes, and jingo. The "Jiminy" comes from "Gemini," which goes back to at least 1664, and which may derive from the Latin Jesu domine. "Jiminy" is sometimes used alone, as in "By Jiminy" or—perhaps a transitional form—"'Oh, geeminy, it's him,' exclaimed both boys in a breath" (Mark Twain, The Adventures of Tom Sawyer, 1876). "Jiminy" also may be finished off in other ways besides the cute "Cricket," e.g., Jiminy Christmas, Jiminy crackers, Jiminy criminy, Jiminy cripes, and Jiminy whiz.

John Bartlett, Dictionary of Americanisms (1848) has this entry for the word, which does not indicate any connection between Gemini and Jesu domine:

JIMINY. By Jiminy! An exclamation. Originally, gemini, or the Castor and Pollux of ancient mythology; names by which the old Romans used to swear.

A search of Early English Books Online returns one early (but quite ambiguous) potentially relevant match for "by Gemini," from Holinshed's Chronicles (1587):

More welcome than Terpsicore was to the towne of Troie. / Sea-faring men by Gemini conceiue not halfe my ioie. / Strong Hercules to Theseus was neuer such delight, / Nor Nisus to Eurialus as I haue in this sight. / Penelope did neuer thirst Ulysses more to see, / Than I poore Norwich hungred haue to gaine the sight of thee.

It seems more likely that the sea-faring men are feeling joy as they use the constellation Gemini to steer by than that they are feeling joy by a euphemism along the lines of "jumping Jehosephat." I haven't yet found the 1664 source that Rawson alludes to.

The earliest use of jiminy as an expostulation that I've found in various searches is from Woman's Will: A Comedy, in The New British Theatre: A Selection of Original Dramas Not Yet Acted, volume 4 (1815):

Old H[arcourt]. I cannot think of it.—For as he is unquestionably my lawful heir——

Lucy. Lawful heir! jiminy, jiminy, how you provoke me! Shall a trifle like this be set in opposition to the force of love? Omnia vincit amor, as the poet says ; and which in English means—that is, as Mr. William, (he was bred at Oxford) informs me—“Love subdues pretty girls,” and this, indeed, he kindly taught me long ago.

On the other hand, the euphemistic use of Jiminy for Jesus seems quite strong in Henry Paul, Dashes of American Humour (1852):

"Jiminy Cranks! Yew ain't agoin to ride in that thing, are yeow? It looks like a patent coffin!" said our Yankee friend, gazing with an air of curious interest at the cab, and crossing to examine the position of the perch. "What a pesky quaie go for the driver to set behind! There's where these things and cabs arn't alike."