[in-fal-uh-buh l]


absolutely trustworthy or sure: an infallible rule.
unfailing in effectiveness or operation; certain: an infallible remedy.
not fallible; exempt from liability to error, as persons, their judgment, or pronouncements: an infallible principle.
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.


an infallible person or thing.

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

1, 2. See reliable. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for infallible

Contemporary Examples of infallible

Historical Examples of infallible

  • Mrs. Turnbull's recipe, infallible for all aches, bruises, and strains.

  • She is not so infallible a markswoman, but that she might shoot at a crow and kill a pigeon.

    Maid Marian

    Thomas Love Peacock

  • Even the simplest of them—to do good to other people—is an instant and infallible specific.

    Pax Vobiscum

    Henry Drummond

  • How, you clever and infallible members of this present generation, do you judge her?

    Priestess of the Flame

    Sewell Peaslee Wright

  • This freshness, however, is not to be taken for an infallible sign of 7.

British Dictionary definitions for infallible



not fallible; not liable to error
not liable to failure; certain; surean infallible cure
completely dependable or trustworthy


a person or thing that is incapable of error or failure
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 infallible

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.

Online Etymology Dictionary, © 2010 Douglas Harper