What does Extern mean?

For the party everyone was allowed to bring an extern.

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

The word separates members or housemates from outsiders at an activity or borrel. As an international you might be the extern at first. Don't worry, it doesn't take long to become one of the group. Also read about normi, dichtgetikt and studentenvereniging.