N. Describes events in which someone injured him- or herself notably while doing something rather stupid. Used like "suicide," but for events which were not lethal (perhaps by a stroke of luck). Generally also only used when the injury was sustained as a consequence of an intentional act, rather than sheer accident following a somewhat amusing streak of bad luck.
Mike committed demicide by attempting to stand up on his bike pedals as he went over the speed bump. He was fortunate to have survived the event, but his friends later wondered if he had a death wish, or was merely being an idiot.