Hip-hop slang, for "I'm out of here." As used, for example, by Nas
," a track from his seminal Illmatic
Mike'll chop it, Mayo, you count the profit
My shit is on the streets that way the Jakes'll never stop it
It's your brain on drugs, to all fly bitches and thugs
Nuff respect to the projects, I'm ghost, One Love
The most common usage is simply, "I'm ghost." It is used only as an adjective.