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[fal-uh-see] /ˈfæl ə si/
noun, plural fallacies.
a deceptive, misleading, or false notion, belief, etc.:
That the world is flat was at one time a popular fallacy.
a misleading or unsound argument.
deceptive, misleading, or false nature; erroneousness.
Logic. any of various types of erroneous reasoning that render arguments logically unsound.
Obsolete, deception.
Origin of fallacy
1350-1400; < Latin fallācia a trick, deceit, equivalent to fallāc- (stem of fallāx) deceitful, fallacious + -ia -y3; replacing Middle English fallace < Middle French
1. misconception, delusion, misapprehension. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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British Dictionary definitions for fallacy


noun (pl) -cies
an incorrect or misleading notion or opinion based on inaccurate facts or invalid reasoning
unsound or invalid reasoning
the tendency to mislead
(logic) an error in reasoning that renders an argument logically invalid
Word Origin
C15: from Latin fallācia, from fallax deceitful, from fallere to deceive
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for fallacy

late 15c., "deception, false statement," from Latin fallacia "deception," noun of quality from fallax (genitive fallacis) "deceptive," from fallere "deceive" (see fail (v.)). Specific sense in logic dates from 1550s. An earlier form was fallace (c.1300), from Old French fallace.

Online Etymology Dictionary, © 2010 Douglas Harper
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fallacy in Culture

fallacy definition

A false or mistaken idea based on faulty knowledge or reasoning. For example, kings who have divorced their wives for failing to produce a son have held to the fallacy that a mother determines the sex of a child, when actually the father does. (See sex chromosomes.)

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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