Don’t worry. You are not meant to be totally oriented in the world. You are being thrown in the deep end of the Lor’Avvu pool, right off the bat, and the larger pieces will hopefully come to you as you read. Well, hopefully not really thrown in the deepest, since the ocean floor of Kaian can be several miles from its surface.
Anyhow, Aop’s kæther Nrfu tells Aop’s father Prguz to stop moving his chin.
It is a commonly slurred combination of ‘stop’ & ‘chin’. As you can guess, it essentially means, “Shut yer pie hole!”