A poosy is when you stick your dick inside a girls arsehole and get juice all over your dick and then stick it back in her pussy - voilà you've made a poosy!
Hey Josh bro, how was your night last night?
Amazing bro! I gave her a poosy!
A combination of the words "poo" and "pussy" to form one word relating the use of the asshole as a pussy. Often, but not necessarily always, used in the case of a mtftranssexual.