This is a variation of the word noob. It is used to describe someone who makes a lot of mistakes that a beginner would make. This is mainly used in Lineage II.
Originated from a videogame that would give you a "killtrocity" award. This was a very rare and amazing event. So whenever a noob does something spectacular, that is a noobtrosity.
"Dude, you just got pwned by that uber noob! What a noobtrosity!" or "Dude, youre so bad, youre a noobtrosity!"