From Discworld & Terry Pratchett Wiki
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.
On the Roundworld, there was a long-running radio comedy show called Round The Horne, which in the best British music-hall tradition relied on a succession of double and single entendres worthy of Nanny Ogg in order to raise laughs. (Dating from the late 1960's, RTH is currently repeated on a never-ending Moebious loop on BBC Seven)
A recurring character was dubious folk-singer Rambling Syd Rumpo (Kenneth Williams) who effectively invented the "zero entendre", where a made-up word with no meaning at all was manipulated into sounding as if it were something horrendous or filthy. Sung in an all-purpose Borsetshire English rural accent, his songs conveyed meaning out of nothing. Imagine the following, sung to the tune of "Clementine":-
"So they hung him by the postern, Nailed his moulies to the fence; For to warn all young cordwanglers That it was a grave offence" (Ballad of the Woggler's Moulie)
TP's use of "moule" echoes Williams' use of "moulie" and for much the same reasons. The line about "they hung him by the postern" is also reminiscent of Carrot's early dialogue with Nobby:-
"Back in the mountains, if a thief was caught, he was hung up by the..."
He paused, idly rattling a doorknob. Nobby froze. "By the what?" he asked, in horrified fascination....
"We hang them up by the town hall! Sometimes for days. They don't do it again, I can tell you" (Corgi PB, p55)