Como surgiu a palavra “Bazinga”?

É uma palavra obscura que o ator de Sheldon ouviu e decidiu que era engraçado de usar. De uma entrevista com a AOL :

"One of the writers, he used to say it, apparently in the writers' room. That's the tale I've heard. But I remember it wasn't in a script. It was one of those moments where we'd work on a scene and then you'd go and take notes from the producers and writers. If I'm correct, it was inserted right before a taping basically. It was like 'That would work in here. What if he said 'bazinga' after that?'" He admitted the audience may not have known what it meant at first, but it caught on for a familiar reason. "The writers liked it but they knew what it meant. But I knew what it meant the moment they said it. It's like 'gotcha", you know, it's just in that energy. There's just something about it. I like it because it's not plain English in a lot of ways and that's very handy."

O Wiki da BBT aponta que foi usado em um episódio antigo de Arquivo X:

In the X-Files episode "Hollywood A.D.," which aired April 30, 2000, character "Chuck Burkes," played by actor Bill Dow, utters the word "bazinga," though it is spelled in the captions as "buzzinga."

Burkes: "Who made this?"
Scully: "We're not sure. Either a forger by the name of Micah Hoffman, or uh, someone else in the vicinity of Jesus Christ."
Burkes: "Buzzinga."

Além da resposta do CDE e se você está procurando a resposta canônica, ela é mostrada em Young Sheldon S02E10 Uma infância atrofiada e uma lata de nozes mistas que Sheldon encontra essa palavra em uma loja de quadrinhos.

Quando ele tenta desenvolver um senso de humor, ele vai a uma loja de quadrinhos que o leva a orquestrar práticas onde ele vê essa frase

If it's funny, it's a Bazinga.

_Fonte

É aí que Sheldon se depara com essa palavra. Ele usa essa palavra quando brinca com seus familiares.

Aqui está o tweet oficial .

De Cinemablend (ênfase minha),

That journey took him to a comic book store, where he purchased several practical jokes. He ended up playing many of them on his family. The practical jokes Sheldon bought were made by the Bazinga Novelty Company. The company's name undoubtedly plays a role, but that is not where the origin of "bazinga!" entirely lies.

The company's tagline holds the key. Bazinga Novelty Company's slogan was "If it's funny, it's a Bazinga!" and that is how Sheldon's catchphrase was born.