In just the past year, however, the etymology took a decisive turn. That's when the American right, faced with widespread opprobrium of the alt-right, made their standard move of constructing a political equivalence: there is also, they argued, an "alt-left". Look at the history of this term, and you'll only see it pop up sporadically over the past few years, for instance on an obscure and stagnant Reddit page. But just this year, you'll find an insurgency of articles like What About the 'Alt-Left'?:
The New York Sun wants no part of [the alt-right] and neither does the GOP. But the Democratic Party has its own fringe for which to answer before Mrs. Clinton has any standing to make a megillah of the “alt-right.” What about the alt-left?
The argument here, quite explicitly, makes two moves that define the entire genre. First, crucially, it de-centers the alt-right by placing it on the "fringe". This isn't necessarily implicit in the original formulation, and in fact the alt-right continues to see itself as "a silent majority or hidden mainstream" in America. While rejecting its claim to popularity, meanwhile, the liberal-left long made a similar argument: as Luis Miranda put it, "the GOP made Trump through years of divisive and ugly politics". There was, that is to say, a widespread consensus that the alt-right was merely just another variety of the American right, just like libertarianism, neoconservativism, paleoconservativism, and so on, and that all were implicated in the same reactionary politics. But this new definition of alt-right implicitly contests that, and its political function here is plainly to distance the "true" right from a damaging label.
Thus, "alt-" went from meaning something like "white capitalist ethnonationalist" to something like "fringe" (a spatial metaphor synonymous with "not-center"). Having reframed the prefix to mean deviation from the center, the American right could then easily reorient it into their own attack. This is the second function of alt's redefinition: to create a slur that can be used to punch left.
For the New York Sun, punching left simply means attacking Hillary Clinton, by creating the false equivalence that she, too, has her own extremists. There is, of course, no equivalence between the fascist base of Donald Trump's movement and any faction of Hillary Clinton's coalition; substantively, the charge doesn't make sense, because if there were white ethnonationalist capitalists supporting Clinton, we'd just call them alt-right Clinton supporters, not alt-left Clinton supporters. But by reconceptualizing "alt-" to mean "fringe", one can get around that substantive problem and apply it to whatever one considers the not-center-left.
This etymology of alt- must be the starting point of any analysis of its role in modern political discourse. Historically, it is laden with the censure that all decent people extend to everything that is detestable about the American right, and historically it is inextricable from the broader evolution of right-wing politics. But rhetorically, it now functions to shield the "center-right" from that critique, and to place in its cross-hairs anything that deviates from the center. Uncoupled as it is from any substantive meaning, defined purely by the centrist metaphor of "fringe", we should not be surprise to see this attack drifting into the hands of centrist liberals - who themselves are always looking for new slurs to wield against the American left.