Unattached, especially romantically, and able to move and act without responsibilities. The “foot” is the bottom of a sail, and a sail that is footloose is free to move whichever way the wind blows. So is a person who is “footloose and fancy free,” at liberty to follow any and all whims. (Such a state sounds enviable, but keep in mind the fable about “The Grasshopper and the Ant.”)