fallacy

What it means

1. Deceptive or false appearance; that which misleads the eye or the mind.

   I no longer believe in happiness, because I see the fallacy of my first belief; and the examination which that induced, has shewn me the fallacy of all. Shew me a heart without its hidden wound.

   1834, L[etitia] E[lizabeth] L[andon], Francesca Carrara. […], volume II, London: Richard Bentley, […], (successor to Henry Colburn), →OCLC, pages 108–109

   1 more example

   Mr Jones expressed great gratitude to the lady for the kind intentions towards him which she had expressed, and indeed testified, by this proposal; but, besides intimating some diffidence of success from the lady’s knowledge of his love to her niece, which had not been her case in regard to Mr Fitzpatrick, he said, he was afraid Miss Western would never agree to an imposition of this kind, as well from her utter detestation of all fallacy as from her avowed duty to her aunt.

   1749, Henry Fielding, The History of Tom Jones, a Foundling, volume (please specify |volume=I to VI), London: A[ndrew] Millar, […], →OCLC

   Synonyms: deception, deceitfulness

2. human sciencesAn argument, or apparent argument, which professes to be decisive of the matter at issue, while in reality it is not; a specious argument.

   Baldridge also showed the "one molecule of blood," usually held to be the stimulus for attracting sharks, to be another common fallacy, since a molecule of blood does not exist.

   1983, Richard Ellis, The Book of Sharks, Knopf, →ISBN, page 163

   Synonyms: logical fallacy, incorrect argument, misargument, non-argument, pseudoargument

In a sentence

I no longer believe in happiness, because I see the fallacy of my first belief; and the examination which that induced, has shewn me the fallacy of all. Shew me a heart without its hidden wound.

1834, L[etitia] E[lizabeth] L[andon], Francesca Carrara. […], volume II, London: Richard Bentley, […], (successor to Henry Colburn), →OCLC, pages 108–109

Where it comes from

1. Latinfallācia“deception, deceit”
2. Old Frenchfallace
3. Middle Englishfallaci
4. Englishfallacy

From Middle English fallaci, fallace, fallas, from Old French fallace (whence -acy), from Latin fallācia (“deception, deceit”), from fallāx (“deceptive, deceitful”), from fallere (“to deceive”).