The word ‘gender’ has been part of the English language since at least 1390. It appears to have started at a grammatical term to distinguish between ‘the classes (typically masculine, feminine, neuter, common) of nouns and pronouns distinguished by the different inflections which they have’ (Oxford English Dictionary). Later, in the 1400s, ‘gender’ came to apply to ‘males or females viewed as a group’ (OED again).