”Cis” is a transphobic term anyway. It means two things about you are “the same”. If you’re “trans”, those things are different. This is just Latin prefixes, for the bigots at the back.
But which two things are the same or different, you may ask? Well this is where we run out of non-transphobic answers. It can’t be that your biological sex is different from your “gender” (leave aside how we define gender for now). Because the concept of a “biological sex” is transphobic. It is impossible to know what it is, or there are 100s, or it’s a spectrum, or trans women are “biologically women”. So that can’t be one of the things that is “trans” in transpeople and “cis” in cispeople.
It is also transphobic to know what my “gender” is without asking me. So you can’t possibly know if I’m cis or not without making an assumption - which is transphobic.
I haven’t heard any other definition of cis, so I am only able to assume it’s now a transphobic term as it assumes a difference between gender/gender identity and some other aspect of a person.
I don’t actually mind it if it just means “not trans”. But then what is “trans” and how do I know I’m not? I am, according to the internet, “under the trans umbrella” because I am not aware of having a gender identity. But according to that definition, almost all the women I know are trans.
(And please don’t draw analogies to “straight” meaning “not gay”. Straight can’t mean that, otherwise bisexuals would be straight. It has a more precise meaning, and so should “cis”, if anyone can tell me what it is.)