The official definition of SCREWBAG is rather vague and can mean many things.
1. Anyone who is not quite retarded enough to be classified as a douchebag and not straight enough to be considered normal.
2. A bag filled with screws of various sizes, commonly used in construction. The one downside to a screwbag instead of a screw box is that if you grab the bag hard it will pierce your hand.....which makes it the opposite of a ballsack (in this context), which hurts like a motherfucker when you squeeze it hard.
3. The male genital region, more commonly called the ballsack. It also can be labelled the SCROTUM, NUTSACK, TOOLBOX, SET, or many other names. This name comes from the fact that sex, or SCREWING someone, involves a ballsack, so if you combine the two you get SCREWBAG.
Dr. Douchenfuck: Look at what Mike is doing over there! He is tipping over the garbage cans into the road!
StupidAss: Yeah, that kid is a screwbag.
The construction worker groped around for the screwbag and seized it, but he squeezed too hard and punctured his hand in multiple places. Luckily it wasn't his ballsack that he seized!
Sarah calmly stroked the long shaft of Greg's 11 inch penis. She loved how erotic his gleaming screwbag appeared in the soft evening light.