Jness is short for “Junk” ness. The only ness these people know is selfish-ness.
Raniere blows through other people’s money without conscience. He’s a douche bag.
So Raniere is inspired by The Mule as a character that he has an affinity for and wants to possibly be like? It fits since Raniere’s been accused of running a mind-control type cult by Rick Ross and psychomanipulative powers are The Mule’s thing.