At the Drop of a Hat

American in origin, with a documented use as early as 1837, well before the phrase became common in everyday speech. The leading theory ties it to 19th-century sporting customs, particularly in Ireland and the American frontier, where dropping a hat was a simple, visible way to signal the instant start of a horse race or a foot race to competitors who might be spread out and unable to hear a shout. A closely related theory points to fairground boxing booths, where a challenger would throw his own hat into the ring as a signal that he wanted to fight the resident champion. Both theories point to the same basic idea, a hat as a quick, unmistakable starting signal, and both have reasonable period support, without one being definitively proven over the other.