"Fire arms are said to hang fire when the flame does not immediately communicate from the pan to the charge." (A pocket dictionary, for military officers, containing a definition of all the tactical terms now in use, with other matter belonging to the art of war, &c. By H.M. Rose, brigade major and inspector in the militia of North Carolina. Raleigh: Printed at the Minerva Press, by Alexander Lucas., 1816.)
Yes, actually this is what happens in ... well, at work, with black-powder muskets, when the sergeant says "Fire!" and the whole rank fires, but one gun goes off a little later than the others for whatever reason. That's why they wait a second, holding the guns in the fired position, before they say "To!" and then you bring them down to make sure they fired or reload, or whatever. (You don't want it going off in a hang fire when you've brought it down to your side.)
I thought this was some weirdness in our little group and didn't know it was used in other instances. Thanks myroblyte!