To be jerked off while sleeping as a revenge attack for being a total piece of shit. Usually the discharge is aimed at the stomach of the victim to make a messy aftermath. When the victim wakes up they will see the cum on their stomach and realized they had been slappied but don't know by who, hence it being a Ghost Slappy. This is in no way gay.
Friend comes home really drunk, pukes on your floor and passes out.
You give him some ghost slappies.
He wakes up and says "Fuck I got Ghost Slappied again!"