What does Jasje dasje mean?

For the borrel everyone had to show up in jasje dasje.

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

It is worn at special events within a student association. A bit more relaxed than a full suit, but you still look the part. If an invite says jasje dasje, leave the hoodie at home. Also read about das and galaplicht.