I’m pretty sure Bussy is slang for a transmasc pussy, as you may know some transmascs retain their vagina as they either can’t afford bottom surgery or just choose not to get bottom surgery as their genitals do not bother them. It’s similar to how transfems who haven’t had bottom surgery for similar reasons often call their penis “Gick” or “Gock” which is a combination of girl and dick/cock. Bussy is a similar slang word, though it’s the combination of the words Boy and Pussy.

(No I will not include a picture for you, use your preferred search engine if you’re curious)