Antigender is an umbrella term for any gender that can only be described as being the opposite of another gender (known as an eugender). For example, antiboy would be the opposite of a male. However, antiboy would not necessarily be female, because male and female are not opposites. Antigender may also be used by people who cannot narrow down their gender, but know they are definitely not a certain gender.
Flag
The antigender flag was coined by beyond-mogai-pride-flags on June 4th of 2018. It has no confirmed meaning.
The second antigender flag was created by u/_potaTARDIS_ on Reddit. It was meant to look like the voidpunk flag. In their words, paraphrased slightly for formatting reasons as well as cutting it down slightly; "The orange and blue are based on the direct inverses on the color wheel of the blue and pink on the trans flag, this direct conflict represents an existence directly contrary to the concept of gender. White represents a positive lack of connection to the concept of gender (bright, airy, carefree). The off-black represents an intentional disconnect from the ideals of gender dictating humanity, but one that is pleasant and not as stark as it may seem. Also represents the reclamation of concepts of depersonalization or dehumanization, especially in regards to the gender construct. A "void" by design. It surrounds the flag, as it is the point in which all these other beautiful and powerful ideas reside. The purple, a mix of blue and red/pink, is the mixing of gender concepts like femininity, masculinity, or androgyny. It represents a playful and happy lack of restraint with these concepts; even if some of these concepts do not apply to you, you may feel no qualms with breaking their rules or feeling them occasionally.It is in the middle, as a circle instead of a line, and warps the concepts around it. This is due to the transgressiveness of it, as well as its refusal to play by heavily gendered rules."
History
Antigender was coined by Tumblr user adhdtobey on an unknown date.