Adjusting one's cock to make a hardon more comfortable. Usually neccessary as the result of a log jam caused by erotic sights such as up skirt or down blouse viewing. The discomfort of the log jam may force a jam buster in public, in which case it has to be done as fast and as discretly as possible.
I got so darn hard at work the other day looking up this girls skirt, I just had to do a "jam buster" to let my cock stand up. I looked around to see if anyone was looking, then as quickly as possible, I put my hand down the front of my jeans and freed my cock.