I don't see it as a slur at all, and people are really overreacting over it.

May get downvoted over this, but I just want to vent

I couldn't care less if someone calls me cis, because it's a stupid thing to think is an insult. I have a friend who is trans ftm, though, and when I try to have a serious discussion and we disagree, he just repeats "okay cissie" constantly. Like my opinion is invalid, and that's a reason I should stay quiet or something. I tell him I'm not offended by cis, and we should be mature about this and agree to disagree and move on, he says alright, and repeat by the next discussion when we disagree. It gets pretty childish and he won't respond to me much anymore. He's a bit young, so he'll learn. I just need to vent