"Cis" is short for "cisgender", ie the opposite of "transgender"

99.999% of earth's population

When I first came across the term, I figured out what it meant based on the context and my interest in history: Cisalpine Gaul (Gallia Cisalpina, now in modern day Italy) was the part of Gaul on the same side of the Alps as classical Rome, while Transapline Gaul (Gallia Transalpina, in modern day France) was on the opposite side of the Alps.