Roget's Thesaurus was available to Joyce in its first edition, 1911.  Its full title is Thesaurus of English Words and Phrases, and it is based on Aristotle's Categories, written in 350 B.C.

    Under the category of "Diffusive Sympathetic Affections" we find this entry for

    Malificent being

    • #913. [Malificent being] Evil doer
       
    • N. evil doer, evil worker; wrongdoer §. 949; mischief-maker, marplot; oppressor, tyrant; destroyer, Vandal; iconoclast|!.
    • firebrand, incendiary, fire bug [U. S.], pyromaniac; anarchist, Communist|!, terrorist.
    • savage, brute, ruffian, barbarian, semi barbarian[obs3], caitiff, desperado; Apache[obs3], hoodlum, hood, plug-ugly*, pug-ugly* [U.S.], Red Skin, tough [U. S.];
    • Mohawk, Mo-hock, Mo-hawk; bludgeon man, bully, rough, hooligan, larrikin[obs3], dangerous classes, ugly customer; thief §. 792.
    • cockatrice, scorpion, hornet.
    • snake, viper, adder, snake in the grass; serpent, cobra, asp, rattlesnake, anaconda|!.
    • canker-worm, wire-worm; locust, Colorado beetle; alacran[obs3], alligator, caymon[obs3], crocodile, mosquito, mugger, octopus; torpedo; bane §. 663.
    • cutthroat §. (killer) 461.
    • cannibal; anthropophagus|!, anthropophagist|!; bloodsucker, vampire, ogre, ghoul, gorilla, vulture; gyrfalcon|!, gerfalcon|!.
    • wild beast, tiger, hyena, butcher, hangman; blood-hound, hell-hound, sleuth-hound; catamount [U. S.], cougar, jaguar, puma.
    • hag, hellhag[obs3], beldam, Jezebel.
    • monster; fiend §. (demon) 980; devil incarnate, demon in human shape; Frankenstein's monster.
    • harpy, siren; Furies, Eumenides.
      • Hun, Attila[obs3], scourge of the human race.
    • Phr. faenum habet in cornu [Lat].

    Of this cornucopia, those phrases associated with "devil incarnate" fit well, especially when adding the word sinful.  Clearly, this paralysis is more than just a realistic illness.  It appears in this phrase to  be more than just the boy's fears.  It appears in its maleficent and sinful aspect to be of mythic proportion, some basic force in the universe.