Defined: The definition of tenable is reasonable, or able to be sustained or maintained. An idea that is reasonable that you can continue to believe is an example of something that would be described as tenable. A situation that can persist for a long period of time is an example of a tenable situation. ADJECTIVE (Your Dictionary.com)
Untenable is defined as something that cannot be maintained or defended. If you have a weak argument that falls apart as soon as anyone expresses any skepticism, this is an example of when you have an untenable argument. ADJECTIVE (Your Dictionary.com)
A position is untenable when it cannot be realistically defended. Your daughter’s argument that she just absolutely has to go to the concert because everyone is going is not realistic. A political position that violates civil rights or oppresses a portion of the populace is untenable.
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that cannot be held, defended, or maintained
incapable of being tenanted or occupied
(Webster’s New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.)
Being such that defense or maintenance is impossible: an untenable position.
Being such that occupation or habitation is impossible: untenable quarters.
(The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.)