Cos wasn't coined for anything even remotely related to sex or gender. It's just the Latin opposite of trans. Which honestly just makes whinging about the term from reactionaries like Musty all the more ridiculous.
My issue is only ever hearing it intended as an insult. I don't really qualify as cis anymore (pansexual now) but when I did I disliked it because I only heard it with that negative connotation.
Because "cis white male" is used pajoratively by certain extreme left groups like it's a problem or something.
The phrase is fine, but saying it like it's a problem is what makes it offensive. Just like any personal characteristic (age, sex, orientation, race, etc), it's not offensive inherently, it's when other groups discriminate based on it that it's a problem. Just because weird bigot people like Elon are saying it like that doesn't mean he started it
Don't give any ideas we're going to have to add another gender exclusive to dipshits like Elon, just known as X or Zed or some fucking dead Alphabet letter