What would be the most accurate term for newbie or noob, namely one that also conveys a dash of sarcasm and a bit of a condescending tone? I guess it should also be slang.
新来的 or 新手 are pretty neutral, 外行 simply means layman.
Well, while I was researching the question I may have found the answer.
Pleco says its 菜鸟, as well as the Chinese version of Newbie on Wikipedia.
One of the other interpretations in Pleco is idiot which may be too harsh，and another is beginner which is neutral and descriptive. But one of the definitions is indeed newbie.
I'm pretty much sure the answer would be 菜鸟, but what the heck, let's post anyway and see if something better comes up.
If you're new to a subject there will often be a 新手指南. You can see this all over: for example in the Chinese language Weibo developer documentation there is a section titled 新手指南。 A 指南 is a "user guide" or "guide book".
(指= to point)(南= although it means south, it is used in the word for compass) I have seen 新手 used perhaps 50 or 60 times to communicate essentially the same meaning as a noob or newbie. 新手, however, is not slang.
It can be used to express newcomer, beginner.
小白 is also a possible translation of newbie.