In Star Trek, a random Redshirt who is taken on an away mission simply for the purpose of getting killed. Can also be used to refer to a superfluous member of any friendship or work group - one who doesn't quite fit in and is often left out, or asked to do things the other people won't.
Nerd #1: Who the hell is that guy talking to Bones?
Nerd #2: Oh, that's just Ensign Dead.
Wes: Ugh, someone threw up in the meeting room at the party last night. It's disgusting - someone's got to clean it up.
Nick: Make Jeff do it; he's such an Ensign Dead.