Is Cis an offensive term?
There appear to be a lot of people in an uproar about being called cis or cisgender. Now, I say "appear" because social media tends to focus on the drama and outrage but it's my hope that the majority of cisgender people are not bothered by the prefix cis.
Cis and trans are simply prefixes. Prefixes are placed at the beginning of words to change, modify, or clarify meaning.
So what does cis actually mean? Cis is the antonym to or opposite of trans. Trans simply means to be on the other/opposite side of (eg. transatlantic, transmountain, transCanada, etc). Cis, being the antonym, means this/same side of; though it's not commonly used in the examples provided for trans, it could be (eg. cisatlantic, cismountain). To be cisgender means that you identify as the same gender you were assigned at birth. That's factual for some people, not offensive.
Cis as a prefix for gender is used as an accurate descriptor, for inclusion, and to show understanding and acceptance of the transgender community. It's not harmful, discriminatory, demeaning, derogatory, etc. to use the prefix cis. When used accurately (the person is cis not trans), it's simply using clear and precise language to avoid misunderstanding, confusion, and assumptions.