This delightfully weird word — meaning nix or nothing — is best known in the US, as the type of wordplay that created it was invented in that country and has always been most popular there. It’s Pig Latin, or igpay atinlay, as Pig Latinists would name it, at one time instead called Hog Latin. Originally a children’s word game known since the 1890s at least, it grew up with them to become fashionable among adults in the 1920s and 1930s. The first known appearance of ixnay in print is in the film script for the early talkie Broadway Melody in 1929.
The basic Pig Latin rules are simple (though there are lots of dialectal variations): if a word starts with a vowel, then add ay to the end; otherwise move the first letter (or pair of letters if they represent one sound) to the end and add ay to it. So imay oingay ootay ostonbay is Pig Latin for “I’m going to Boston”.
The trick has never quite vanished. The film The Lion King included the line ixnay on the upidstay — don't say hyenas are stupid, the hyenas are listening. In Young Frankenstein, the hero complains in the hearing of the monster about the monster's rotten brain; Igor hisses, Ixnay on the ottenray!
Yet another example:
Something glinted and caught his eye. He hissed to the nearest kid, “Ixnay on the ottlebay!” The eight-year-old, squirming in his unaccustomed clothes, flushed and tucked the busted bottle farther out of sight.
Gladiator-at-Law, by Frederik Pohl and C M Kornbluth, 1955.
Ixnay is from nix, nothing, a slang term that was imported from German nichts at the end of the eighteenth century.
It’s uncertain to what extent Pig Latin was used as a cant or argot designed to keep criminal discussions private, but ixnay seems to have become part of the vocabulary of the hard-boiled hoodlums of the period:
I would’ve done the job only he wanted to put me on the nut, so I says ixnay, gimme the geetus now.
Los Angeles Times, 8 November 1931, in an article with the title Underworld “Lingo” Brought up to Date. Nut means a debt; geetus is money.
The term was carried abroad in the detective novels of the time, perplexing British readers until greater understanding of American mores in wartime and afterwards clued us all up and allowed ixnay to appear in British English, though as an exoticism.