adjective

lacking sense or clear, sound reasoning.

an illogical fear of the supernatural

irrational,unreasonable,unsound,unreasoned,unfounded,groundless,unjustifiable,unjustified,incorrect,erroneous,wrong,invalid,spurious,faulty,flawed,fallacious,unscientific,inconsistent,unproved,specious,sophistic,casuistic,absurd,preposterous,untenable,implausible,impossible,beyond belief,senseless,meaningless,nonsensical,insane,ridiculous,idiotic,stupid,foolish,silly,inane,imbecilic,crazy,off beam,way out,full of holes,daft,barmy,backasswards