They either don't know what it means, or when you use it they roll their eyes and go on with their day, finding you silly and a try-hard but they aren't overly offended.
Like... They might be less likely to offer to get you a beer, assuming you're probably annoying in other ways too, but it doesn't rise to the level of declining a beer you offer them and finding out.
It's not that "cis" is an insult.
It's that the kind of people who use it tend to be insulting in other ways, usually to people in the majority or people in positions of power.
As in, cisgender is aligned in the normal way to biology. Cisnormative is most people.
Cis is from Latin, and is the antonym to trans.
It's sorta the equivalent of instead of saying normal and abnormal, one would say adnormal and abnormal. Like... There's nothing inherently wrong with being abnormal, and changing normal to something that means normal to mask what it means seems silly, but, you know... Seems a weird thing to be actually offended by if you're normal.
It's sorta like the "black lives matter"/ "all lives matter" thing.
People that say "black lives matter" don't believe "all lives don't", and people that say "all lives matter" couldn't truthfully say that if they believed black lives didn't.
But BLM peeps usually think "ALM" is used to minimize unique hardships in the black community and ALM peeps think "BLM" is used to make something that's a universal, if lopsided, problem like police brutality a racial thing or something.
People that use the word "cis" don't automatically think something negative about straight people, though. It depends how it's used in context and it's only sometimes used as a perjorative, not all the time.
Like... You can say "x is the fault of cis people" like an antisemite might say "x is the fault of the Jews", but you also might use it in a non offensive way to just substitute for straight or whatever.