Newbie, Nub, Noobie - these are all internet (l33t) slang words.
For definitions of slang words, use this:
http://www.urbandictionary.com/1.
Newbie - Someone new to something. A first timer.
2.
Nub - a internet acronym, similar to "noob". HOWEVER, whilst a NOOB is someone who is lame/idiotic because he is new, a NUB is someone who has had the opportunity to learn something new, but never did, and is therefore idiotic/stupid.
3.
Noobie - A person who is new at playing an online game and/or is bad at it.
No one knows when these terms originated, but I remember when I was 12, playing Quake2 online (I'm 19 now

) and people were using them. So it probably started soon after the net began being used for online games.
Also, as I always say, skill is not necessarily represented in how MANY kills you have, but HOW you play the game. This really applies to Enemy Territory (what I play now a days, along with Trem) but XP/SCORE means NOTHING if you don't know what you're doing and why.