"Thắt lưng"

Translation: Belt

[Raphael_SyBo]

So literally a "tie back" or "something that ties the back" haha ^_^

[TehVanarch]

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.

[ckhadung]

That's correct. It literally means the purpose of its existence.

[Andy499355]

Wiktionary says this is chiefly used in the North of Vietnam. Can anyone confirm/refute?