underboo Posted in U Best definition underboo A person who is obssessed with Undertale, and will go as far as write fan fics and etc. Derives from Undertale and Weeaboo. You are such a fucking Underboo! How satisfied are you with the definition? Looking for underboo meaning? Delighted that you discovered the greatest determine on our web site! Recently this definition was actually observed 47 times.