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[in-fal-uh-buh l]
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  1. absolutely trustworthy or sure: an infallible rule.
  2. unfailing in effectiveness or operation; certain: an infallible remedy.
  3. not fallible; exempt from liability to error, as persons, their judgment, or pronouncements: an infallible principle.
  4. Roman Catholic Church. immune from fallacy or liability to error in expounding matters of faith or morals by virtue of the promise made by Christ to the Church.
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  1. an infallible person or thing.
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Origin of infallible

First recorded in 1375–1425; late Middle English word from Medieval Latin word infallibilis. See in-3, fallible
Related formsin·fal·li·bil·i·ty, in·fal·li·ble·ness, nounin·fal·li·bly, adverbnon·in·fal·li·ble, adjectivenon·in·fal·li·bly, adverb

Synonyms for infallible

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1, 2. See reliable.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for infallibly

Historical Examples of infallibly

  • Certain causes will infallibly be succeeded by certain effects.

    Old-Fashioned Ethics and Common-Sense Metaphysics

    William Thomas Thornton

  • Be it so; only, bethink you that one of us will infallibly be killed.

    A Hero of Our Time

    M. Y. Lermontov

  • Ergo, again, ours must infallibly top the markets of the world.

  • The hand of God is raised against it and will infallibly strike it.

    Dona Perfecta

    B. Perez Galdos

  • You will infallibly suspect me of having chosen them thus on purpose.

British Dictionary definitions for infallibly


  1. not fallible; not liable to error
  2. not liable to failure; certain; surean infallible cure
  3. completely dependable or trustworthy
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  1. a person or thing that is incapable of error or failure
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Derived Formsinfallibility or infallibleness, nouninfallibly, adverb
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

Word Origin and History for infallibly



early 15c., from Medieval Latin infallibilis, from in- "not, opposite of" (see in- (1)) + Late Latin fallibilis (see fallible). In reference to Popes, attested from 1870. Related: Infallibly.

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Online Etymology Dictionary, © 2010 Douglas Harper