What does SG mean?

"SG!" he shouted, and immediately had to do his own shotgun.

Dutch student slang. This is a Dutch word. You will hear it a lot at Dutch universities, student associations and in student houses.

Call shotgun yourself and you really have to do one: saying it is doing it, no exceptions. So think twice before you shout it at a party. A shotgun means punching a hole in a can and drinking it in seconds. Also read about shotgun a beer, shot and jägerbomb.