Clopin Trouillfou is the king of beggars and in many versions he is the king of the entire Paris underworld.
Clopin is his given name. It comes from an expression that means to “to hobble along” or Marcher clopin-clopant. Clopin itself comes from the word clopiner meaning to limp. Unlike Frollo, whose given name also mean limp, here it reference the fact that Clopin is king of people who fake ailments for money as well as he himself who pretends not to have a leg.
Trouillfou is a compound of Trouille which means fear with a strong connotation, as in “scared to death,” and fou which means fool. Basically it mean scared crazy. This name suggests that he either is crazy or his presence makes people crazy, like Gringoire or his legion of beggars.
I always thought that Clopin was a variation on Clovis (first king of France). And Louis is a variation on Clovis. Clovis/Louis-King of France, Clopin-King of beggars/gypsies…etc.