Someone who claims to be any gender other than male or female.

It might be a real gender or it might not exist, I don't fucking know.
Other "genders" are often joked about in memes. That is where the word comes from.

It comes from the "There are only two genders," meme. Anything other than the first two generic genders would be a memegender.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.