What is tatty-bye?

As an AI language model, I'm not aware of any specific term called tatty-bye. However, the term tatty-bye appears to be a British slang term that means goodbye or farewell. It can be considered informal and casual and is often used in informal or friendly settings, such as among friends, family members, or acquaintances. The term tatty-bye is likely derived from the phrase "Ta ta" or "Tatty" which is an informal way to say "Goodbye" or "See you Later". The term Tatty is also used in the UK to mean old or worn-out things.