[edit: I thank everyone for their comments and time. A lot of very interesting opinions and view points. Unfortunately also a lot of things that went away from the actual answer. So I’m thinking maybe this thread can be closed without deleting it?]
The more I hear people talk about it who aren’t cis-het men, the more I hear criticism about the concept. But so far, I’ve only heard people say that it’s stupid, that it’s not a thing, that it’s men’s own fault etc. But I’ve yet to understand where that criticism comes from. I don’t want to start a discussion on whether or not it’s real or not. I just want to understand where the critics are coming from.
Cis effectively means not-transgender, so born as exacly the same gender you identify as. ‘Het’ then means heterosexual, making cis-het someone who is either completely male and into women or completely female and into men.
I have definitely heard cis in terms of cognition so I don’t think its specific to sexuality.
It is a prefix that isn’t specific to gender (I don’t know of a particular use in sexuality though that doesn’t mean there isn’t one), but in other uses that I know of, it isn’t used by itself as a descriptor of an aspect of a person’s identity, but as part of some other word. It basically means the opposite of trans (as a prefix, so not just “cisgender means someone that isn’t transgender”, but anywhere that the prefix trans- could be used, for example, when talking about spacecraft visiting the moon, the space farther away from earth than the moon is is sometimes referred to as translunar space, and conversely, the space between the earth and the moon can be called cislunar space). In general, if one is talking about people, especially if it’s just used by itself with nothing else attached, it just refers to everyone other than transgender people.
Similarly, carbon=carbon double bonds in fatty acids can have the free hydrogens either on the same side or on opposite sides of the double bond, and are known respectively as cis or trans fatty acids.