A prefix which is usually followed by an animal infamous for having an exceptionally large or small penis. The meaning is that the person to whom the statement is directed has a penis of a similar size as that of the animal he is "hung" like.
I.E. He's really huge, or really tiny! Depends on the animal used.
Girl 2: Lucky bitch... I'm gonna dump Gerald, he's hung like a hamster!
Other examples include the sperm whale, donkey, horse-fly, bull... I could go on!