Well, it originally came form online games, when a 'newb' or 'newbie' (i.e. someone who was new to the game) was someone you wanted to avoid playing with, because they ruined the game by being rubbish at it. That evolved to 'noob' or 'n00b' (so as to emphasise it among the other text).
However, through extensive use, it evolved from specifically meaning someone that was new, to someone who was naive or inexperienced. This probably came about as gamers started to use it on the Internet in non-gaming contexts, so when someone asked a stupid quesiton, for example, they would be branded a 'n00b!'.
Then it came to be used as a wider, general insult, when someone had made any kind of gaff. It really isn't related to tenure anymore.
Hope that helps darling