For originating of the fannish use of the term, we apparently have to think the fairly recently deceased Craig Hinton, who (according to his obit in DWM), having created the term, cheerfully agreed when it was used to describe much of his own writing.

Since then the meaning has drifted and generalised somewhat, from "attempts to explain way plotholes that no one else really cares about *in fiction*", via doing so in on-line discussions, to just generally discussing anything in an obsessive fashion, to please oneself rather than with any serious prospect of edifying others.

Wikipedia article: http://en.wikipedia.org/wiki/Fanwank

Of course, just 'cos the originator of a term didn't intend it to be used nastily, doesn't mean that it won't be... =:o\