fallacies

noun pl. fallacies

1. Obsolete deception
2. aptness to mislead; deceptive or delusive quality: the fallacy of the senses
3. a false or mistaken idea, opinion, etc.; error
4. 1. an error in reasoning; flaw or defect in argument
   2. Logic an argument which does not conform to the rules of logic, esp. one that appears to be sound

Origin: ME fallace < OFr < L fallacia, deception, artifice < fallax (gen. fallacis), deceitful < fallere, to deceive: see fail