In a non-ordinary state of consciousness, especially one in which one is removed from physical reality; in a trance; out of it; out to lunch; dreaming while awake. Probably derived from 'The Twilight Zone', a TV program dealing with fantasy, science fiction, the occult, etc. Also 'zoned out', 'zoning'.
The driver of the truck that hit me seemed to be completely zoned -- the cops took him away, and left me sitting on the curb.
-- I'm by myself, but I'm not alone / I spend a lot of time in the Twilight Zone. -- (James Taylor)
When you are high (off weed) to the point that you literally keep "zoning" out. If this occurs while you are around other people, usually you will not participate in the conversation and instead sit and listen. Or not listen at all and just think. It can be very peaceful, but it can also make people wonder if you are ok and make you seem awkward.