This article is from the Feminism References FAQ, by Cindy Tittle Moore firstname.lastname@example.org with numerous contributions by others.
This term was "invented" by the radio/tv host Rush Limbaugh. He
defines a feminazi as a feminist who is trying to produce as many
abortions as possible. Hence the term "nazi" - he sees them as
trying to rid the world of a particular group of people (fetuses).
This term is of course completely without merit, but there's the
definition of it FYI. [CTM]