The term was coined by Tumblr user genderless-gibberish on July 9, 2018.
The flag was designed by Tumblr user Nooonbinaryyyy on July 9, 2018. The first three stripes come from the colors of the thistle flower. The white stripe represents the absence of gender. And the green comes from the agender flag.
The term was named after flowers that are often referred to as thistles. The suffix -ian was added at the end.