From Urban Dictionary:
2. noob
647 up, 144 down
comes in two variations, noob and newb.
Noob is meant as an offensive term towards someone who is not nessicarily new, and quite often is directed at someone that is |somewhat| experienced but at the same time stupid/disruptive and/or totally incapable of learning.
A noob is almost always someone who thinks they're cool but obviously aren't. Also used in 1337speak.
see also: n00b, nub, ch00b, n00bo, n00blet.