This term was used in the movie "Get Him To The Greek". It is a concoction of mostly marijuana laced with opium, heroin, peyote, methadone, crushed up ecstasy, some of it is unidentifiable, and hint of angle dust and possibly a few other things.
I'm pretty sure this was actually cleverly named for Jeffrey Miron, a Harvard economist who is lobbying for the legalization of ALL drugs.
"Nobody's scared of a jeffrey." "Jeffrey's just a bloke down the street." ~ Get Him To The Greek
From the sounds of what all is in it, I think it would be really stupid to smoke a jeffrey. You'd probably die instantly.