The state assumed by some people for up to 15-20 hours a day. It is the opposite of being vertical and consists of sleeping, laying down, general inactivity and lack of motivation to do anything.
Usually occurs during a bout of food-poisoning or a night on the beers which can lead to severe pains and abnormalities relating to excretion. Upon going to the toilet for a dump, rather than fall vertically from your backside it will spray, or release horizontally. Called 'doing a horizontal'.