Just as "trans-" means on the other side of. It's literally just the opposite of trans.
The term cisgender has its origin in the Latin-derived prefix cis-, meaning 'on this side of', which is the opposite of trans-, meaning 'across from' or 'on the other side of'. This usage can be seen in the cis–trans distinction in chemistry, the cis and trans sides of the Golgi apparatus in cellular biology, the ancient Roman term Cisalpine Gaul (i.e. 'Gaul on this side of the Alps'), and Cisjordan (as distinguished from Transjordan). In cisgender, cis- describes the alignment of gender identity with assigned sex.
What are the origins of the word "cis"?
https://en.wikipedia.org/wiki/Cisgender
Just as "trans-" means on the other side of. It's literally just the opposite of trans.