Boyish is an umbrella term for any sort of male-aligned, man-aligned, boy-aligned, or masculine-aligned gender, and it was made as an easy-to-understand alternative to labels such as mingender, mascgender, and miaspec.
Boyish can also be an identity on its own for one who feels like they are sort of a boy, not quite a boy, not fully a boy, almost a boy, a little bit of a boy, or any other sort of variation of that could be described as boyish. The girl version is girlish.
Boyish is not the same as boyishgender, they are two completely different genders.
The gender boyish stems from the term boyish, which is used to describe an aspect of something that is traditionally masculine.
The term was coined by an anonymous Wikia user on February 23rd, 2021.
The blueish-green on the flag represents the way boyish people may be sort of, but not fully, a boy. The purple represents femininity, masculinity, and the mixing of the two, and the white stripe represents the union of all identities that could be considered 'boyish,' just as white is a mixture of all colors.