"Prejudice, stereotyping, or discrimination, typically against women, on the basis of sex" (Google)
"prejudice or discrimination based on sex; especially : discrimination against women" (Merriam-Webster)
"prejudice or discrimination based on sex; behavior, conditions, or attitudes that foster stereotypes of social roles based on sex" (Wikipedia)
I think these are all perfectly acceptable "plain English" definitions of "sexism".
Definitions are nothing but arbitrary convention anyway--my point is that when you explicitly unpack the feminist definition of "sexism", seemingly semantic arguments like Mr. Klabnik's are packing in a lot of assumptions that deserve to be unpacked. Generally, I don't favor definitions that pack in tendentious assumptions for political purposes.
As you noted, those definitions are all arbitrary, but needless to say they leave out the entire history of the origin of the word, an origin which seeks to critically describe both individual instances of sexism as well as the systemic, social factors of sexism. It should be noted though that the Wikipedia definition does include something about more than just individual instances of sexism.
I disagree that there are extra meanings being packed into the word sexism beyond the meanings you cited. That you are unaware of the origins and issues that go into sexism doesn't remove the meanings of the word. To be fair, no mainstream outlet or publication tends to talk about things at that length and level for a variety of reasons, many of which are due to systemic sexism, but don't confuse common understanding for the only understanding. Common understandings that lack depth or more rigorous information are what contribute to a common understanding that promotes racism, sexism, and other issues.
That post seems to unpack more or less the same assumptions I'm unpacking:
"A running theme in a lot of feminist theory is that of institutional power: men as a class have it, women as a class don’t"
"What this imbalance of power translates to on an individual level is a difference in the impact of a man being prejudiced towards a woman and a woman being prejudiced towards a man"
Or as I called it: "the notion that everything in society is systemically biased in favor of men, at the expense of women", leading to the conclusion that "it doesn't matter as much when a man faces sexual discrimination".
Fine. We agree that the same assumptions are being packed into the feminist usage of the word "sexism". My argument is that these assumptions need to be called out and defended in this forum. If this were a feminist forum where everyone could be reasonably assumed to have already accepted those assumptions already, perhaps the implicit jargon would be more appropriate. Furthermore, the act of imposing this jargon is a backhanded way of imposing the assumptions behind that jargon.
I totally agree about the inaccessibility of a lot of feminist literature, including things on sexism. Talking about the meaning and reasons behind sexist incidents and sexism, as steveklabnik did, is important and esp. so in places where basic, fundamental info is not known.
There's nothing Orwellian about that, though, because we are talking about information that hasn't been made available or accessible for others rather than taking terminology and changing its meaning to suit a political agenda diametrically opposed to what the original term stood for. In fact, that mass media has perpetuated sexism and feminism as things that do not have a systemic basis or ignoring that they do is the more Orwellian thing going on in US society.
As for imposing assumptions, the notion that using terminology correctly and explaining and examining that terminology is somehow backhanded in a discussion or argument is silly. How else are we going to get better understanding without using actual terms and the ideas behind them?
Your argument seems to be that since feminists invented the word, it's pretty much up to them what it means. Fine. My response: that definition still packs in assumptions unfairly and it would be better to abandon the word entirely, at least in forums where the assumptions packed into that word's definition are not universally held premises.
Since you claim that there is a redefinition, what exactly do you think sexism meant in the first place?