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1.
When someone new to YouTube finds videos that are very famous and commonly known among experienced YouTubers, they think no one else have seen the video, and send/talk about it to everyone they know.
Friend 1: Dude! I just saw this HILARIOUS video! It's called "Chocolate Rain"! I dont know if you've ever seen it before, but you should totally check it out!

Friend 2: Are you serious man? That video has over 36 million views. Everyone has seen that video already. Man, your a TubeNoob.

by browntrousers April 27, 2009
 
2.
Someone who can't ride on the train properly and keep on falling into other people.
on the tube
Dude: Watch as I ride this train with no hands!
Friend: Why? You're just going to fall into me again, you're SUCH a Tube Noob
by Ruberducky June 10, 2010