Webster's English Dictionary

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fallacy

fal.la.cy \'fal-*-se-\ n [L fallacia, fr. fallac-, fallax deceitful, fr. 
   fallere] to deceive - more at FAIL obs  1a: GUILE, TRICKERY  1b: deceptive 
   appearance : DECEPTION  2a: a false idea  2b: erroneous or fallacious 
   character : ERRONEOUSNESS  3: an argument failing to satisfy the conditions 
   of valid or correct infere nce