If you visit ニコニコ動画 or any Japanese message boards often you are bound to see comments like ニコ厨 or 東方厨. Does anyone have good idea how did this originate and what do they mean?
I believe the following theory, but I have never tried to back them up with an evidence:
It originates from a slang 中坊 (ちゅうぼう). It means “junior high student,” but often with an indication that the speaker looks down on the student he/she is talking about. (The usual word for “junior high student” is 中学生.)
On a BBS, calling someone 中坊 would be just another way to insult the person who is referred to; because the speaker does not know the age of the person referred to, it can only mean that the speaker thinks the action of the other party is childish as if he/she were a junior high student. But because 中坊 is a slang, popular IMEs cannot translate ちゅうぼう to 中坊. Because of this, the word 厨房 (kitchen) with the same pronunciation but completely unrelated meaning was used in place of 中坊 on a BBS. Later the abbreviation 厨 came into use.
ニコ厨 means a childish Niconico Video freak, and 東方厨 means a childish freak of games and/or game characters made by Touhou Project. But the meaning “childish” is sometimes ignored and they are often used to just refer to a Niconico Video freak or a Touhou Project freak.
Also it may be worth noting that ニコ厨 has the same pronunciation as ニコ中, which is an old-fashioned abbreviation for ニコチン中毒 (“dependence on nicotine/smoking”), and therefore ニコ厨 may be a pun for some people. But I do not know if this fact played any role in the etymology of the word ニコ厨.