They were called Minutemen because they could be ready to fight in a very short time after hearing that the enemy was coming.

Exactly like Kole said. They would be ready to fight within a minute's time of receiving the order, hence the name. I got my degree in Computer Science at the University of Massachusetts in Amherst, Massachusetts. UMASS' athletic teams are called the Minutemen after this historic group. :) You can see their logo here: http://cache.milesplit.com/user_files/324547/106041/umass861.jpg

from our friend wikipedia: Some towns in Massachusetts had a long history of designating a portion of their militia as minutemen, with "minute companies" constituting special units within the militia system whose members underwent additional training and held themselves ready to turn out rapidly ("at a minute's notice") for emergencies, hence their name.