It's a Latin word meaning "judge" or "avenger."
When Georges Franju embarked upon his remake of Louis Feuillade's Judex in 1963, he took for granted that his audience was familiar with the story of the 1916 serial or that they wouldn't mind if he had to make giant leaps of logic to get to the next plot point. Either way, he worked with screenwriters Jacques Champreux and Francis Lacassin to boil the story of Judex down to its essence, discarding some characters and bulking up the roles of others. They also added a number of outré touches, like the elaborate birds masks worn by the revelers at the soiree unscrupulous banker Farvaux (Michel Vitold) throws to celebrate the engagement of his daughter Jacqueline (Edith Scob, returning from Eyes Without a Face) and the 20th anniversary of his company, which was founded on blackmail and fraud. Then there are the black hoods worn by Judex's men, who are all fairly interchangeable as a result.
The same cannot be said for the title character himself. This Judex (Channing Pollock) is as much a magician (the actor's actual trade) as he is a master of disguise (his main one being Farvaux's longtime secretary, who literally knows where the bodies are buried) and a moral crusader. Lacking the time to go into his motivations for specifically targeting Farvaux (which is why the characters of his brother and mother have been excised), the film instead has Judex make oblique references to "the Panama Scandal" as he goes over his misdeeds with Jacqueline while her daughter's governess, Marie (Francine Bergé), eavesdrops. After she's dismissed along with the rest of the servants, Marie hatches a plan to return to Farvaux's empty chateau with her accomplice Morales (Théo Sarapo) to retrieve some incriminating documents. (Taking his cue from Feuillade's Les Vampires, Franju has Marie don a black body stocking and mask for his nocturnal operation, with Morales following suit, although his outfit isn't nearly so skintight.) This turns out to be one of several occasions where Martie and Morales wind up abducting Jacqueline, but their general ineptitude keeps them from being very effective kidnappers.
In case you were wondering, defective detective Cocantin (Jacques Jouanneau) is also in play, although his role is mostly limited to regaling Jacqueline's daughter with Lewis Carroll's Alice stories (because that's her name) and reading from a Fantômas book at one point so Franju can complete the Feuillade hat trick. And this time his late-arriving love interest is a circus performer named Daisy (Sylva Koscina) who happens onto the scene of the thrilling finale just after Judex has scaled a building where Marie and Morales are holed up with Farvaux and gotten himself captured. It's a good thing, too, because if they had waited for Judex's hooded minions to show up, he would have been a goner. Instead, the villains get what's coming to them and Judex gets the girl. Say, what's French for "win-win"?