Hello, official "noob police" here! :)
'Noob' simply means that you're inexperienced or lacking skill in a particular game or activity. In other words, the player who referred to you as such was either hubristic upon killing you, or was saddened that you killed them and felt the need to belittle your abilities. Try not to take insult to it, since it isn't offensive by any definition. Instead, see it as a compliment at best, and an urge to improve at worst :)
~Noob police, out