at the drop of a hat

Example: “She'll travel anywhere at the drop of a hat.”

Origin: Traced to early-to-mid-1800s America, where dropping a hat was a simple, visible way to signal the instant start of something with no verbal countdown needed. Exactly which activity gets credit is unsettled: one account points to fairground boxing, where a challenger threw his hat into the ring to announce he wanted to fight; another to horse racing, particularly in Ireland, where a dropped hat started the race; a third to the American frontier, where the same signal is said to have started footraces, horse races, and impromptu brawls alike. All three share the same essential picture — a small, ordinary object doing the job of a starting gun, and the earliest citation for the phrase itself dates to 1837.