technically the slur form would be "cissy"

but language reveals its very artificiality: the only word that starts with it is cisalpine, which is distinctly artificial--people with a roof over their heads aren't called cissients; people not moving aren't being cisported; plastic isn't an electric cisducer (also Transylvania used to be called Ultrasylvania and it should be changed back)

it'd be like calling most people singletons (also people shouldn't have to prove whether they're a twin to say they are one, and I am a political twin which is valid and not subject to internalized geminophobia, which some twins have when they say things like "I define myself by more than just my twinhood" or "stop using your twinhood as a cudgel" or "talk about literally anything else for the love of God")