characterized by faithlessness or readiness to betray trust; traitorous.
deceptive, untrustworthy, or unreliable.
unstable or insecure, as footing.
dangerous; hazardous: a treacherous climb.

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

Synonyms for treacherous

Antonyms for treacherous

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

Related Words for treacherously

unscrupulously, deceitfully, perfidiously, difficultly, unsafely

Examples from the Web for treacherously

Contemporary Examples of treacherously

  • Trying to build a one-state reality in the name of “inalienable rights and realpolitik” is treacherously dishonest.

    The Daily Beast logo
    Another Reason Dani Dayan is Wrong

    Elisheva Goldberg, Sahar Segal

    August 3, 2012

Historical Examples of treacherously

  • 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



betraying or likely to betray faith or confidence
unstable, unreliable, or dangeroustreacherous weather; treacherous ground
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.

Online Etymology Dictionary, © 2010 Douglas Harper