So literally a "tie back" or "something that ties the back" haha ^_^
The full form is "dây thắt lưng" where dây refers to stringy objects. There's also "dây nịt" which is more commonly used by Southerners.
That's correct. It literally means the purpose of its existence.
Wiktionary says this is chiefly used in the North of Vietnam. Can anyone confirm/refute?