(BDSM, especially in roleplay) A malebottom (i.e. submissive partner), defined not by junior age, but by his obedient role, especially when that implies him being subject to spanking and/or other punishment by the dominant "top".
“Congrats to muh boi Mikel on the new baby Boi… He finally made me a Grandad.”
“Hat boi staging is modeled on conventions of Chinese opera.”
“In the 20th century there was a movement to loosen the rigid structure of hat boi and to reduce the high proportion of Chinese loanwords that makes the operas difficult for the ordinary Vietnamese to appreciate.”