Also "hop up." Used when referring to airsoft
guns. It is best described as the backwards spin that the gun is designed to put on the bullet, making the bullet spin the wind below it during "flight" so that it doesn't descend as fast. A lot of airsoft guns have too much hopup, so the bullets might travel upwards at times.
Crap, this gun sucks. It has too much hopup.
Sweet, that gun has just the right amount of hopup to shoot straight.