Term in reference to someone who has been blessed with an extra large penis, in the eyes of the beholder. In short, someone with a bigger than average dick.
In most areas, this is about both the length, and girth of the penis.
It also can refer to how the penis, hangs low when flaccid, due to its size.
“Hung like a horse” is a common slang term, to denote someone very well endowed.
One can have a rather lengthy penis, but not thick, or wide, and still be considered to be hung.