A Ginetta is a term used for someone who is very stupid and also a major buzz-kill. They have an uncanny ability to ruin a good joke or even a pleasant situation. A Ginetta is also a person that misuses slang or tries to act cool when they really just end up looking like a poser.
Guy: Oh my gosh that kid totally killed the joke!
Guy 2: Ya, she's such a Ginetta.