[e12e]

Webster's define (the relevant meaning of) gender, as:

"b : the behavioral, cultural, or psychological traits typically associated with one sex

(...)

Origin: Middle English gendre, from Anglo-French genre, gendre, from Latin gener-, genus birth, race, kind, gender — more at kin. First use: 14th century"

So it would seem gender has started out as a term that groups "indivuduals that are of a kind" - not necessarily based on (biological) appearance, but just as much on behaviour/social grouping.