What does Boxing Day have to do with boxing?
U.S. history, one of a class of militia available for immediate service (i.e. "ready in a matter of minutes"), 1774. As the name of a type of ICBM, from 1961, so called because they could be launched with very little preparation.
Armed American civilians who were active in the Revolutionary War and in the period just preceding the war. They were named Minutemen because they were ready to fight alongside regular soldiers at a moment's notice. The Minutemen of Massachusetts were especially well known. (See Battle of Lexington and Concord.)
Note: During the cold war, the name “Minuteman” was given to a United States missile held ready for launching in the event of a nuclear attack.
: Youth gangs use minors as gun runners, calling them ''minutemen'' because if they're caught they're out (of jail) in a minute (1990s+ Street gang)