UNSC stands for: United Nations Space Colonies. It is used mostly in the Halo books, but you may catch some people saying it in the Halo video games.
A deranged Marine- OMFG!! THAT GRUNT JUST BLEW UP ALL THE UNSC COLONIES!!
Master Chief- Relax, you numskull. What you saw was what is going to happen to you when you finally find out that you have half a dozen needlers sticking in your side. Oh, and they were launched by a grunt too.