The use of the word cis has its roots in an obscure Usenet group; it's genesis (apparently) rooted in a desire for more inclusive language for trans folks (the notion that "gender" Vs "transgender" was too othering).

It hit Tumblr like a train in the 2010s, and became a symbolic phrase in trans counterculture. "Cisgender" was less than popular with non-trans people, as it robbed them of the illusion of normality and turned the word "gender" into a social trap.

It later found derogatory use in the phrase "cissy" (a counter for the popular derogatory term "tranny").

It's a fun word with an interesting history, and it has helped contribute to the wider acceptance of trans folks.