Una opción más. Otras expresiones en español que también pueden seguir esta misma estructura (de repetición) y denotan ironía, mofa, escepticismo:
- ¿ganga?, ¡menuda ganga!
- ¿premio?, ¡menudo premio!

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Who invented the sh- thing
Shm-reduplication is a form of reduplication in which the original word or its first syllable (the base) is repeated with the copy (the reduplicant) beginning with shm- (sometimes schm-), pronounced /ʃm/. The construction is generally used to indicate irony, derision or skepticism with respect to comments about the discussed object:

He's just a baby!
"Baby-shmaby".[1] He's already 5 years old!
What a sale!
"Sale, schmale".[2] I'm waiting for a larger discount.
The original word can be a noun, but also an adjective:

"Whenever we go to a fancy-schmancy restaurant, we feel like James Bond."[3]
The reduplicated combination belongs to the same syntactical category as the original.

https://en.wikipedia.org/wiki/Shm-reduplication

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Studies in Etymology and Etiology: With Emphasis on Germanic, Jewish ... David L. Gold
Pág. 614-620.

It's a Yiddish construction “X + shmX”.

[...]It would presumably be Yiddish maline 'hide-out'. Thus, we would have Yiddish maline -> *maline-shmaline (a nonce formation) [...]

Expressions:old shmold; cold shmold; sake shmake; fancy-shmancy packages; old world, shmold world; ...

Expressions are more often than not used in pejorative contexts. A missing link between the Yiddish and the Czech word could possibly be a Polish reflex of the Yiddish word, namely: Polish melina ‘1. hide-out of criminals > 2. storeroom for stolen things’.

The word can be found in the Internet as a word used by young people. The notion of a melina is generally associated with low quality of both rooms and products.

https://books.google.es/books?id=l015C5vm1XkC&pg=PA616&dq=go...


Gold shmold. At least that’s what the leprechauns are saying these days. It turns out the pot of gold at the end of a rainbow isn’t nearly as fun as eating the rainbow itself.

https://www.pinterest.com/pin/484137028668948211/

[...]Horris was no help. He began sniffling, sneezing, and snorting because
of the hay. And then he began muttering to himself that he was hungry and this was
all just a stupid waste of time. “Gold shmold,” he mumbled. “Rather have a
cheeseburger, any day.”
Payne sighed. He was on his own. Not that he expected great things from
Horris, but a little cooperation would have been nice. After all, he did intend to split
the gold with Horris, 50-50… well, maybe 60-40… how about 70-30? He would
have to see. No matter, he’d find what he was looking for by himself.[...]

The If Bridge - Smashwords.pdf

Si bien se puede tomar como un idiomático como lo propone John, es más una frase simple de decepción.