The entire thing is arbitrarily defined. You only need to experience an unqualifiable, unquantifiable Thing to be a word that didn't exist ten years ago ("transsexual" was the prior term, but postmodernists have falsely abstracted gender from sex) and if you don't experience Thing then you're a different word that also didn't exist ten years ago. Sex has existed since before fish grew legs and first walked on land. Gender is literally a word related to "genre" and its definition is completely situational.
What I'm saying is there's no rulebook for this stuff. Tumblrinas literally make up new nouns to describe how people they don't like are evil.
The term "
transgender" was coined in 1965 for much the same reason that "gender" for a long time was conflated with sex (as a classification, the older meaning), to avoid the usage of "sex" to refer to fucking or to sexual behavior or sexuality, which a term like "transsexual" implies, even though, to quote from the book in question, "sexuality is not a major factor in primary transvestism."
Curiously enough, the term "
transsexual", brought to English in 1949 as "transsexualism", a translation of Hirschfeld's 1923 German coinage "Transsexualismus", was popularized by Harry Benjamin (after whom the WPATH institute and standards were named for decades, until late 2006) in
The Transsexual Phenomenon in 1966, but the year later he started using terms like "gender identity".
Sometimes, today, the distinction is made that a transsexual is a transgender person who seeks surgical reassignment; well into the 1980s, describing oneself specifically as a "transsexual" was necessary to obtain trans-related medical treatment.
As for the notion of gender as a distinct, but related, notion relative to sex, that was probably influenced by research into transsexualism (the "distinct" bit) and the long-standing relationship between grammatical gender (originally just a term for different "kinds" of words, similar to "classes" and "declensions") and sex, with people of the male and female sex being referred to with consistently different grammatical genders, and in Western languages maybe
one other grammatical gender existing. (There are languages in which
grammatical gender is not aligned with sex in this way, like "animate" vs. "inanimate".) In English especially, with distinct grammatical genders only existing for third-person singular pronouns and derived words, the choice of "gender" as a less-taboo word for "sex" was easy to make, and even today, the use of "gender" as a not-strictly-reproductive classification of humans is in accordance with historical usage, even though the notion of a "gender identity" not in accordance with one's physiological sex was difficult to believe before the 20th century.
I will admit, though, that it's likely that transgenderism originated with humans, or at least there's no way I'm aware of to detect transgenderism in non-human animals; there is a way to detect behavior in other animals that under the
cisgender assumption (term coined in the early 1990s and then popularized by Julia Serano in 2007) would be clearly
homosexual, but if a male
mammal regularly undergoes
lordosis, even though the parsimonious interpretation is that it's homosexual behavior (less parsimoniously, evidence that the mammal "is" homo- or bisexual), it could be evidence of transgenderism and we would just have no way to tell the difference.
All of the dates but one that I mentioned here pre-date Tumblr; Serano's
Whipping Girl was published the same year Tumblr was founded (2007), but she had used "cisgender" and the less-common "cissexual" earlier, and both had been used by others, again, in the early 1990s.
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With that said, the Tumblrites who disparage "truscum" are the
real scum.
