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See more synonyms for treacherous on Thesaurus.com
  1. characterized by faithlessness or readiness to betray trust; traitorous.
  2. deceptive, untrustworthy, or unreliable.
  3. unstable or insecure, as footing.
  4. dangerous; hazardous: a treacherous climb.
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Origin of treacherous

1300–50; Middle English trecherous < Anglo-French, equivalent to trecher deceiver (trech(ier) to deceive + -er -er2) + -ous -ous. Cf. French tricheur trickster
Related formstreach·er·ous·ly, adverbtreach·er·ous·ness, nounun·treach·er·ous, adjectiveun·treach·er·ous·ly, adverbun·treach·er·ous·ness, noun


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

Related Words


Examples from the Web for treacherously

Contemporary Examples

Historical Examples

  • These are treacherously attacked one night by Finn's men, 1073.



  • Yet of this, my only remaining comfort, I was treacherously and cruelly deprived.

    Gomez Arias

    Joaqun Telesforo de Trueba y Coso

  • I will inform that lovely wife how treacherously you have acted.

  • He jumped backward, exclaiming that I had wounded him treacherously.

    The Memoires of Casanova, Complete

    Jacques Casanova de Seingalt

  • I had you at my mercy when I was treacherously struck down from behind.

British Dictionary definitions for treacherously


  1. betraying or likely to betray faith or confidence
  2. unstable, unreliable, or dangeroustreacherous weather; treacherous ground
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Derived Formstreacherously, adverbtreacherousness, noun
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 treacherously



early 14c., from Old French trecheros (12c.), from trecheur, agent noun from trechier "to cheat, trick" (see trick). Figuratively, of things, from c.1600. Related: Treacherously; treacherousness.

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Online Etymology Dictionary, © 2010 Douglas Harper