Deriving from a "vegetative" state often used in the
medical field, it is a state of total physical apathy, usually on a couch or bed with music or in front of a TV; influence of drugs aids but is not essential to the meaning of the term.
The earliest media use of the term "veg out" originates from the 1990 film
Pretty Woman where
Julia Roberts says: "Let's watch old movies all night... we'll just veg out in front of the TV."