This is your regularly scheduled reminder that ‘cis’ literally means ‘on this side’. It’s a term with Latin origins that has been used in scientific language for 135 years, and it’s not a ‘slur’.
The opposite of ‘cis’ is ‘trans’, which means ‘across’, and ‘transgender’ is just one of its many uses. You have probably eaten trans fats, for example, or perhaps you have been on a ‘transatlantic’ flight.
Ain’t science and language grand?