Moules

According to The Dictionary of Eye-Watering Words, a game of skill and dexterity, involving tortoises.

As it also appears in a fearsome oath sworn by initiates of the Elucidated Brethren of the Ebon Night, it may also be a euphemism for a part/parts of the body which would cause exquisite pain if abruptly removed from its/their habitual surroundings and shown to the four winds.

Eiher that, or the Supreme Grand Master is playing an intellectual joke on underlings who are too credulous or lazy to go and look up the word in a dictionary.