The word 'cis' is a Latin prefix. It means "on this side of [object]". It's used in descriptions of space, time, & a few other concepts.
Source: 5 years of Latin plus the Lewis & Short Latin Dictionary (the best there is): https://www.perseus.tufts.edu/hopper/text
In Latin, you might find Caesar describing different parts of Gaul using these prefixes, as an example: "Galla Cisalpina" refers to Gaul that was on the same side of the Alps as Rome, while "Galla Transalpina" referred to Gaul on the opposite side of the Alps from Rome.
Again, just an example of how the prefixes worked in the original language.
The terms were never used by the Romans as a slur, they were just descriptive. Today, they're used in chemistry, math & biology to describe things like molecular structures; and of course they're used in human biology to describe whether or not someone's gender ID matches their sex as assigned at birth.
In no case are the terms 'cis' or 'trans' slurs in the least. They never have been. They are only descriptors.
The only reason why someone would object to being called 'cis' is because they think that being 'trans' is something bad, they think 'trans' is a slur, they don't believe gender is mutable or fluid, they have a very rigid binary idea of sex & gender, & they don't want to be described in contrast to a concept or people they despise.
The very idea that gender is fluid is deeply offensive to these people; it literally disgusts them. Here's a paper on that, about the influence of moral frameworks; check the links to other papers for more: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6562335/
Declaring 'cis' a slur is also a kind of DARVO, which stands for Deny, Attack, Reverse Victim & Offender - something abusers do in order to deflect focus & blame away from themselves & turn it onto their targets. In this case, the targets are trans people & those who support them. Transphobic bigots turn 'cis' into a slur because they're pretending THEY are the victims, not the aggressors.
Here's a bit about DARVO. I've also been informed that my Lewis & Short link didn't work, so give me a sec to rustle up a better one & I'll post that, stay tuned. https://dynamic.uoregon.edu/jjf/defineDARVO.html
OK see if this link works. There should be a box somewhere on the right side of the page labeled "Dictionary Entry Lookup"; put the term 'cis' in the box and see what comes up. https://www.perseus.tufts.edu/hopper/text