2 a : a false or mistaken idea b : erroneous character : ERRONEOUSNESS

From Dictionary.com:

6 results for: fallacy

Dictionary.com Unabridged (v 1.1) – Cite This Source

fal·la·cy /ˈfæləsi/ Spelled Pronunciation[fal-uh-see]

–noun, plural -cies.

1. a deceptive, misleading, or false notion, belief, etc.: That the world is flat was at one time a popular fallacy.

2. a misleading or unsound argument.

3. deceptive, misleading, or false nature; erroneousness.

4. Logic. any of various types of erroneous reasoning that render arguments logically unsound.

5. Obsolete. deception.

[Origin: 1350–1400; < L fallācia a trick, deceit, equiv. to fallāc- (s. of fallāx) deceitful, fallacious + -ia -y3; r. ME fallace < MF] —Synonyms 1. misconception, delusion, misapprehension. Dictionary.com Unabridged (v 1.1) Based on the Random House Unabridged Dictionary, © Random House, Inc. 2006. American Heritage Dictionary fal·la·cy (fāl'ə-sē) n. pl. fal·la·cies 1. A false notion. 2. A statement or an argument based on a false or invalid inference. 3. Incorrectness of reasoning or belief; erroneousness. 4. The quality of being deceptive. [Alteration of Middle English fallace, from Old French, from Latin fallācia, deceit, from fallāx, fallāc-, deceitful, from fallere, to deceive.] The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2006 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved. Online Etymology Dictionary fallacy 1481, "deception, false statement," from L. fallacia "deception," from fallax (gen. fallacis) "deceptive," from fallere "deceive." Specific sense in logic dates from 1552. Online Etymology Dictionary, © 2001 Douglas Harper WordNet fallacy noun a misconception resulting from incorrect reasoning WordNet® 3.0, © 2006 by Princeton University. Kernerman English Multilingual Dictionary (Beta Version) fallacy [ˈfӕləsi] noun — plural ˈfallacies a wrong idea or belief, usually one that is generally believed to be true; false reasoning Example: That belief is just a fallacy. Arabic: مَظْهَر خادِع، مُغالَطَه Chinese (Simplified): 谬误 miu4 wu4 Chinese (Traditional): 謬誤 ㄇㄧㄡˋ ㄨˋ Czech: omyl, klam, blud Danish: vildfarelse; fejlslutning Dutch: misvatting Estonian: eksikujutelm Finnish: harhakuvitelma French: sophisme German: der Trugschluß Greek: πλάνη Hungarian: téveszme Icelandic: röng hugmynd, villa Indonesian: kekeliruan Italian: sofisma Japanese: 誤った考え ayamattakangae Latvian: kļūda; maldi Lithuanian: klaidinga nuomonė, klaidingas įsitikinimas Norwegian: feilslutning, villfarelse Polish: błędna wiara Portuguese (Brazil): falácia, sofisma Portuguese (Portugal): falácia Romanian: sofism Russian: заблуждение Slovak: klam Slovenian: zmota, zabloda Spanish: falacia, sofisma Swedish: vanföreställning, villfarelse Turkish: yanılgı See also: fallacious Kernerman English Multilingual Dictionary (Beta Version), © 2000-2006 K Dictionaries Ltd. American Heritage New Dictionary of Cultural Literacy, Third Edition fallacy 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. [Chapter:] Conventions of Written English The American Heritage® New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.

Beginning with Aristotle, informal fallacies have generally been placed in one of several categories, depending on the source of the fallacy. There are fallacies of relevance, fallacies involving causal reasoning, and fallacies resulting from ambiguities.

Recognizing fallacies in actual arguments may be difficult since arguments are often structured using rhetorical patterns that obscure the logical connections between assertions. Fallacies may also exploit the emotional or intellectual weaknesses of the interlocutor. Having the capability of recognizing logical fallacies in arguments reduces the likelihood of such an occurrence. [However, if you do recognize and point this out to your opponent, and if they are looking to ‘win at all cost’ and not interested in working toward a resolution of the problem, they most likely would rely on a raised tone of voice to try to extend their position, or they would attempt to ‘Muddy the Water’ with additional non-sequiturs. You will get blue in the face attempting to point out the inconsistency of such an illogical argument.]

A different approach to understanding and classifying fallacies is provided by argumentation theory; see for instance the van Eemeren, Grootendorst reference below. In this approach, an argument is regarded as an interactive protocol between individuals which attempts to resolve a disagreement. The protocol is regulated by certain rules of interaction and violations of these rules are fallacies. Many of the fallacies in the list below are best understood as being fallacies in this sense.

So, that being said, you’d only look like an idiot if you hadn’t checked your facts.

1. a deceptive, misleading, or false notion, belief, etc.:

‘That the world is flat was at one time a popular fallacy.’

2. a misleading or unsound argument.

3. deceptive, misleading, or false nature; erroneousness.

4. Logic. any of various types of erroneous reasoning that render arguments logically unsound.

So in your example, using the word “fallacy” is appropriate.

But in a debate you have to prove that their assumption or statement was incorrect in the first place.

* a misconception resulting from incorrect reasoning

What you say instead is that there is evidence that the statement may not be fully correct

“Marijuana is not bad for you” is not a fallacy. it is a claim.

you will sound like an idiot if you call it a fallacy. you should just present a counter-claim such as : “Marijuana is bad for you.”