A person who cannot be defined as male or female by sight, so it has to, by default, be determined that they are both and neither. Effectively creating a Schrodinger's Cat paradox paradigm.
"is that a beast of a box-y woman in cargo pants or a really snazzy fat dude with mad moobs in a pink body warmer?" can't tell? Schrodinger's Twat.
Applies to really butch lesbians, effeminate Emo/Goth/Scene boys, and other hormone imbalanced individuals that skirt the grey area between the fine lines of the Venn diagram that is gender, making them indefinable from their counterparts.