There are 2 levels of DNS:
Catagory 1: The most common application. Do not sleep with the aforementioned unless drunk and intoxicated, which gives you a ready-made excuse if your friends call you out.
Catagory 2: Indefinite. Any kind of sexual encounter with this person is heavily frowned upon, and is likely to result in ridicule. Designed to stop you from sleeping with a heffer or duff when you're lashed and feeling horny. A person may also be a catagory 2 if sleeping with him/her would cause waay to many problems/trouble, e.g. your best friends boyfriend/girlfriend.
Josh: "As long as you're careful bro, she's a cat. 1 DNS. She only looks good after half a bottle of vod".
Tom: "Don't worry about me dude, i've got this!"
Josh: "Haha, it's all you man."
1a. Also, it can be modified with new different words, but convey the same meaning. (i.e. DSL=Dank Sausage Links)
1a. Did you see that girl's ass? It was straight up DSL!