What does Zeester mean?

"In bed she was a real zeester," people said about her.

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

Picture a starfish lying on its back. That is the idea. It is used for the person who stays passive during sex. Not a compliment, and mostly heard in gossip. Also read about matras and airfryer.