So I have been working out for about a year. I have downloaded a ton of videos and books regarding fitness/ body building. I haven't really came across an exercise that works out the back of arms...
About arms.
Big Biceps
Huge Triceps
Not much muscle at all on the backside/under of arms.
The triceps are a muscle with three ("tri") heads. The underside of your upper arm is one of those heads. Work your triceps through the full range of motion and you will eventually get some size there.
Hand position plays a part in deteriming which head gets worked primarily, but initially just work the basics until you see what needs to be addressed specifically.