If something is carried out At The Drop Of A Hat it is carried out immediately. The phrase can easily be traced to the 19th century, though it is probably much older. At this time, sporting referees – especially in boxing and horseracing – would raise their hat into the air to alert competitors to be ready to begin, and then drop it to signal that the event was underway. An event like this was seen as being started at “ the drop of a hat”.