The word transgender is a recent addition to English. In conversational use as early as the 1960s, “transgender” entered the dictionary in the early 1990s. Trans- is a Latin prefix meaning “across or beyond.” Gender shares the same Latin root as genus. As a classifier for male and female, “gender” replaced “sex” in the 20th century. This was a trend started by feminist writers who wanted to highlight the biological attributes of males and females separate from their social characteristics.
While the word transgender is very new, the idea of behaving outside a traditional gender identity role is quite old: A whole level of meaning to Shakespeare’s plays, often in the form of double entendre revolves around the men dressed up as women to act in female roles.