Well-hung is a joking, informal way of saying a man has a large penis. It is a body word, and it belongs to the same register as crude jokes among friends — locker rooms, comedies, dirty songs. It is not a medical term and it is not polite. If you say it about someone at work, or to someone you do not know very well, you have crossed a line, even though the word itself sounds mild and old-fashioned. Notice that it is nearly always said about a third person, not to someone's face, and usually with a laugh in the voice. The "hung" part is just the ordinary past participle of hang, used the way English often does for something that hangs down from the body. You will also meet plain "hung" doing the same job — "he's hung" — which is even more slangy. Quite separately, well-hung is used about meat, especially game birds and beef, which are left hanging for days or weeks to age and become tender; a butcher or a cookery writer will use it with a completely straight face, and that is the only setting where the word is neutral. The two uses do not get confused in real life because the food one always sits next to words like beef, pheasant, or venison.