The word is a threat because it linguistically separates biological sex from socially constructed categories of “woman” and “man.” That gender is a social construction undermines heteronormativity, critical to defending patriarchal sex roles and procreation.
Cisgender originated on Usenet in alt.transgender in 1994.
I Coined The Term ‘Cisgender’ 29 Years Ago. Here’s What This Controversial Word Really Means.
https://www.historians.org/research-and-publications/perspectives-on-history/may-2017/tracing-terminology-researching-early-uses-of-cisgender
Well, damn.