The sad thing is, it's a fake Tumblr word not recognized by any dictionary. Not OED or MW or anywhere else. Tumblr has coined a lot of interesting words though; "genderfluid" is another that started as a joke, then had people adopting it as sexuality for real.
Official language recognition generally lags the real world by a significant amount. A word can be in use for many years before it even gets a consideration into a dictionary. Dictionary's reflect the world, not dictate it.
I’m not sure when this supposed joke began, but from what I found[1] with a quick search “genderfluid” was used at least as early as 1994 and not as a joke. The same source cites an appearance in Urban Dictionary in 2007, but the entry doesn’t appear anymore and the Wayback link is currently returning 503 errors for me.
