TERF is a slur aimed at women who know sex is binary, immutable and sometimes matters. Oddly enough seldom used for men who know these things - looks like trans activists actually do know the difference between men & women when it comes to put-downs.
It is an acronym for Trans Excluding Radical Feminist - a misnomer because radical feminists exclude men, however they identify, and include women, whether they want to be women or not, and because few TERFs are in fact radical feminists. There’s nothing particularly radical about a reality-based approach.
”Cis” isn’t an acronym. It’s a prefix from chemistry & geography, and the opposite of the prefix trans. Transalpine Gaul was Gaul on the other side of the Alps from Rome, Cisalpine Gaul was the same side of the Alps from Rome.
When the term transsexual started to morph into transgender the activists started defining people who don’t identify as the sex they’re not as “cis”. This is to make us out to be a subset of the sex we are, and to make it seem as if people who are factually not that sex appear part of that sex class.
This is why Gender Identity Theologians can be very insistent that trans & cis are adjectives, and to be written trans woman, so as to imply we’re just all different types of women.
It often appears in the racist rhetorical formulation that “black women are women, so trans women are women,” aka hiding misogyny behind their racism.