Offgender is a gender in which someone feels most closely represented by a certain gender, but is not that gender. For example, an offboy would feel very close to the description of "boy" yet is not a boy. It can be likened to what the color off-white is to white
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.