A mistake made when using an English figure of speech. Usually used when the mistake is commited by Ziva David, a character in the television show NCIS. However, this word is never used in the actual show, only by fans of the show.
"Hey, mind if I take a batnap?"
"Um, dude, that was a Ziva-ism."
In the world of NCIS fans, a Ziva-ism is typically an idiomatic mistake made by the character Ziva David. She is somewhat foreign and is still trying to grasp the American language. American idioms drive her up the hall as she said.
Ziva: "I would hate to be misunderstood."
Fornell: "And does that happen often?"
Ziva: "Once in a blue lagoon."
Ziva used blue lagoon instead of blue moon, so this is a Ziva-ism.