Abimegender is a xenogender which is profound, deep, and infinite. It is best described as when one mirror is reflecting into another mirror, creating an infinite reflection. Abimegender can be used by itself, or as a prefix to describe another gender (eg. abimegirl, abimeagender, abimefluid, etc).
The flags were created by Tumblr user pride-flags-for-us on or before August 23, 2015. They are supposed to resemble one mirror reflecting into another, having the reflections be endless. The color green is used on the abimegender flag because it is not only a color associated with non-binary genders, but also because it's the color that mirrors retain when reflecting into each other.
The prefix "abime-" comes from the French word abîme, meaning "abyss," which is further derived from the Latin prefix a-, meaning "without," and byssos, meaning "bottom."