When some one is cheated that person got horns. So if the person in question got cheated a couple of times or just to emphasize, you say that he got a bag of horns.
Hey you "bag of horns"!
Shut up you "bag of horns"
You see a guy's girlfriend
on a disco cheating on him with a couple of guys and then the other day you see him and say.
-Hey what's up with that bag?