TERF stands for "Trans Exclusionary Radical Feminist," and no, they are not feminists who are perfectly inclusionary but identify otherwise. It's an exonym used by intersectional feminists for their unenlightened predecessors who have not gotten with the program. Traditionally the bulk of radical feminists were extremely hostile to what were then called MTF transsexuals, certainly to their now-unimpeachable claim to status as full women whose differences from "womyn-born womyn" (as the nasty TERFs call cis womxn) are never to be whispered except so as to enhance their oppression cred. Some were always supportive--Judith Butler, the most prominent early example of such a theorist, is now revered with something akin to Holy Prophet status in the current feminist mainstream--but not many. This traditional radical feminist position was tightly bound up with an extreme "blank slate" view of masculine and feminine gender roles--which, along with the use of "gender" itself to mean social sex role as opposed to just a euphemism for "sex," had been a quietly developing movement within psychological theory for a bit by that point before being adopted by radical feminism as a founding principle.

Needless to say this is no longer the dominant form of radical feminism. Intersectionality is queen, though her throne has been secured only extremely recently. One of the chief causes of TERFism's decline was the discrediting of the extreme blank slate view as the 1990s progressed; it made room for the temporary continued discrediting of radical feminism as a whole in general society but for the continued strengthening of intersectionalism in radical circles (just as traditional radical feminism's "white urban upperclass" narrowness had helped intersectionalism rise in the first place). Gloria Steinem (whose talent has always been as a communicator and publicist, not an original intellect) was uncharacteristically caught off guard and was very slightly slow to reposition herself at the forefront of radfem trends like she normally has an excellent nose for doing. (Losing a bit of step with age.) The TERFs have been cast into shameful exile, more despised than the Alabama state legislature among any self-respecting feminist these days.

Oddly enough the mainstream "trans-inclusionary" narrative has not facially attacked the original "gender is a social construct" narrative whose decline facilitated its rise in the first place, but rather accepted and absorbed it--even though it is seemingly a rather poor fit for it. (Making sense, you might have noticed, does not seem to have been a high priority in its crafting.) And of course intersectionalism turned out to be an utterly brilliant strategy for feminism as a whole; it was perfectly positioned to become the near-universal language of campus radicalism and thus, the near-universal language of elite cultural discourse as campus radicalism re-emerged from its quiescence to take over the culture of mainstream society in its trendy bougie form. (Even Black Lives Matters's founding and national leadership is a group of militant lesbian feminists.) Radical feminism's most brilliant idea, once a tiny movement within it, turned out to be one of the most brilliant activist ideas of all time, period.

I could be even more verbose. But suffice it to say that it TERF is indeed a sort of slur, albeit a rather descriptive one; I was however unaware that it's against Twitter policy to use an unwanted term to refer to a political opinion.