>>7 You rely on dictionaries that are only updated every few years far too much. :)
Language is a living thing, new words appear all the time. One needs to have a definition now if one wants to participate in a discussion that uses new words.
Urbandictionary uses a voting system that pushes bad definitions down and good definitions up. Not perfect, but it works remarkably well.

What anout Wiki? Wiki is also made by random people.
"When it is necessary to refer to someone who is anonymous, it is typically necessary to create a type of pseudo-identification for that person. In literature, the most common way to state that the identity of an author is unknown is to refer to them as simply "Anonymous."" http://en.wikipedia.org/wiki/Anonymous

In the often-used sentence "Anonymous does not forgive", Anonymous is obviously a noun. A better debate here is the questionable use of -es to make a plural out of Anonymous. Shouldn't it be "Anonymice" instead? :)