"Jessy" is not a commonly recognized slang term in British English. It's possible that you've encountered a misspelling or a regional term not widely used.
It's worth noting that "Jessy" could be a nickname for someone named Jessica or Jesse. However, it's not considered slang in this context.
If you can provide more context or examples of how you've heard the term used, it might be possible to figure out its meaning.
