A Malawian cultural/religious (usually men's) secret society that is a central part of the Chewa tribe. Members perform a dance called "Gule Wamkulu" or the "big dance" at weddings, funerals, initiations and other celebrations or gatherings where they take the form of an animal or "beast" and communicate messages from the ancestors to the village. They may also kidnap people and do other naughty stuff like extort money.
The term can be used in reference to the society or the masked dancers themselves.
He dumped the whole christian thing these days, he's gone back to his roots and joined Nyau
Behave yourself, stop acting like a Nyau