I'd like to know the spelling of a word, a synonym of unknown

I'm used to watching American TV Series all the time. I watch them with Italian subtitles, so I misspell many words. Many of them are not so difficult to figure out, but there's one that I just cannot figure out what is. I have tried to find it in many ways (Google Translate and many other sites) with no results.

The word sounds like jen though (in Italian I would write it gen do). It seems to mean "unknown" in police jargon: when the police find a corpse with no identity, they call it a ‘jen though’.

What can this word be?

When a person’s identity is unknown (which is often the case when a dead body is found, before the body is identified) or must be anonymised, but the person still needs to be entered into some kind of system that requires a name, placeholder names are often used.

The most common placeholder names are John Doe for males and Jane Doe for females (as mentioned in Håkan and Jim’s comments to the question itself); sometimes just Doe is used on its own for persons of unknown sex. What you’re describing sounds like Jane Doe. Another common version has Richard Roe and Jane Roe, which is most famously found in the common name of a US Supreme Court decision regarding abortion, known as Roe v. Wade (Jane Roe being here the plaintiff’s anonymised name).

There is a lengthy Wikipedia article about these placeholder names, with lots of additional information.