At the drop of a hat

“At the Drop of a Hat” is an expression that means to do something without delay, warning or much preparation.

In the 19th century, a hat was often used to indicate the start of a race or a fight. A hat would be dropped or swept in a downward arc and participants would begin.This was particularly in usage in the American West.

If you do something at the drop of a hat, you do it immediately without really stopping to think about it:

"Do you think I can just meet you at the drop of a hat?"  Though in a usage such as "People will file lawsuits at the drop of a hat these days," there must have been some thought beforehand.