I’m not eloquent with words, OccasionallyIncomplet but this explanation by the poster BahHumbygge perfectly explains why using the word cis, in terms of women or men, is bullshit.
Cis
BahHumbygge
Cis and trans mean on this side of and on the other side of, respectively.
So, in organic chemistry, you can have different isomeric versions of the same molecule... that is the layout of the molecule can have differing configurations. Imagine those 3D stick things from chemistry lessons... if you place the red atoms in opposite corners, that's a trans molecule; if you place them on the same side, that's a cis molecule.
It can also apply to other spatial arrangements of things eg cisalpine/ transalpine. Cisalpine refers to on this side of the alps, from the perspective of Italy... so on the Italian side of the alps.
However, it is a fabrication that you can describe a spatial relationship between an empirical phenomenon (biological sex) and sociological invention (cultural gender).
BahHumbygge
Thinking about this further and what the organic chemistry analogy means for women is really offensive and essentialist from a radical feminist analysis, and paradoxical.
If you imagine the pair of functional groups arranged around a double carbon bond in a molecule being in either a cis or a trans configuration (see attachment), the pairing relationship must be of the same things (so say either ethyl groups, or methyl groups or hydrogen atoms or green blobs if you're not a chemist).
So what the transactivists are saying with regards to sex and gender is that femaleness (the state of being of the class that has large immotile gametes & can gestate, ie hardware) is exactly the same as femininity (the set of traits, socialisation and cultural stereotypes that have come to be associated with the female sex, ie software).
Not only is this essentialist bullshit, what feminists have been fighting for decades to overcome... that girls and women must like pretty sparkly things, be sweet and deferential and be socialised into the caring and subservient role in society, it is non-sensical. Transwomen are at the same time claiming that femaleness and femininity are exactly the same thing by applying the cis-trans model to sex & gender, yet contradicting themselves by being biologically male and claiming femininity. Femininity cannot be both inextricable from femaleness AND possible to transgender into from a male bodied person. Transgenderism is an impossible chimera. You need an empirical thing on which to hang the concept of gender. Once you attempt to cross genders, you invalidate the very claims you were making in the first place.
Female =/= femininity. Therefore there cannot be any ciswomen and there cannot be any transwomen.
Transwoman is an oxymoron. Ciswoman is an oxymoron. The only way to be a woman is through biology.
(image source: www.chemguide.co.uk/basicorg/isomerism/geometric.html)