Used mostly in online games, reffering a person who has performed or said a stupid or wrong thing. Also used to taunt noobs and newbs.
the word "Baka" in Japaneese simpley means "Stupid".
When you are really really good with something, like the Japs. The word is very often used for people who spend most of their spare time on one specific task and thus becoming very good at that task
Boy1: What did you do this weekend?
Boy2: I played Guitarhero
Boy3: Is that all?
Boy1: You must be real Japanees