treacherous

UK: /ˈtrɛtʃərəs/ US: /ˈtrɛtʃərəs/ ,(trech′ər əs)

WordReference Random House Learner's Dictionary of American English © 2025

treach•er•ous /ˈtrɛtʃərəs/   adj. 

1. of or relating to treachery:
   the treacherous tyrant who stabbed his friends in the back.
2. deceptive or unreliable.
3. unstable or insecure;
   hazardous:
   treacherous footing on the icy slope.
4. dangerous:
   a treacherous wind nearly capsized the boat.

treach•er•ous•ly, adv. 
treach•er•ous•ness, n. [uncountable]

WordReference Random House Unabridged Dictionary of American English © 2025

treach•er•ous  (trech′ər əs), adj. 

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.

• Anglo-French, equivalent. to trecher deceiver (trech(ier) to deceive + -er -er²) + -ous -ous. Cf. French tricheur trickster
• Middle English trecherous 1300–50

treach ′er•ous•ly, adv. 
treach ′er•ous•ness, n. 
Synonyms

• 1. unfaithful, faithless, treasonous.
• 2. deceitful.

Antonyms

• 1. loyal.
• 2. reliable.

Collins Concise English Dictionary © HarperCollins Publishers::

treacherous / ˈtrɛtʃərəs/ adjective

1. betraying or likely to betray faith or confidence
2. unstable, unreliable, or dangerous

ˈtreacherously adverb

ˈtreacherousness noun