You can think of 'cis' and 'trans' as meaning roughly 'this side' and 'other side'.
Before “cisgender” became a widely publicized term, about the only time I ever heard the term “cis” was discussing Cisalpine versus Transalpine Gaul in ancient Rome. (Cisalpine Gaul being northern Italy and Transalpine Gaul southern France, more or less - the parts of Gaul that were on the same side or the opposite side of the Alps from Rome).