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.
@Impious_Jade
True.
We are long past the point of looking for reasonable justifications of Musk's statements and actions.
All people can use a little constructive criticism once in a while. He's powerful enough to ignore or insulate from that.
Combined with an autistic person's single mindedness and rejection sensitive dysphoria (I'm sure he is ADHD from what he's said) and it can lead to worrying results. We need more Gretas and fewer Elons.
@DanielCoopreal YES. This is indeed another tragic thing, that it both hits Musk personally, & likely impacts a lot of families with similar qualities.
I am NB but not trans. I can only imagine that every time a trans child hears "cis is a slur" from their own parents, it's yet another rejection or denigration of what they are themselves, in a backhand way. It's a parent stating indirectly that they don't think "trans" is OK, whether it's the word or another person's lived existence.