Hip-hop slang, for "I'm out of here." As used, for example, by Nas on "Represent," a track from his seminal Illmatic LP.

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.