treacherously

American  

[trech-er-uhs-lee] / ˈtrɛtʃ ər əs li /

adverb

1. in a way characterized by faithlessness or betrayal; traitorously.

   Darius of Persia was treacherously wounded by two of his own officers.

2. in a way that is deceptive, untrustworthy, or unreliable.

   This peak looks treacherously easy, but it has seen a recurrence of avalanches.

3. in a way or to a degree that is dangerous; hazardously.

   He refused to go faster than 25 mph on the treacherously winding and narrow Wildwood Road.

---

Other Word Forms

• untreacherously
  adverb

Etymology

Origin of treacherously

treacherous ( def. ) + -ly

Explanation

When you act treacherously, you betray someone. If you tell everyone in school your best friend's carefully guarded secret, you've behaved treacherously. Acting treacherously hurts or deceives another person, or betrays a secret or promise. A little boy might treacherously give away his sister's hiding place during a game of hide and seek — and his sister, in turn, might treacherously announce to some older kids that he's scared of the dark. In both cases, a secret has been betrayed. Treacherously comes from the adjective treacherous, with its Old French root word, trechier, "to cheat or trick."

Example Sentences

Other notable singers included bass-baritone Robert Frazier as the Rev. John Hale, who realizes the truth too late, and soprano Kresley Figueroa as the treacherously weak-willed Mary Warren.

From The Wall Street Journal • Mar. 23, 2026

The playing surface was treacherously slippy with snow piled up around up around the touchlines and conditions barely playable.

From BBC • Sep. 11, 2023

In a statement late Friday, he said he was ready to find a compromise but “they have treacherously cheated us.”

From Washington Times • Jun. 24, 2023

In a clunky new production of Wagner’s “Lohengrin” by the director François Girard, the tenor Piotr Beczala seemed almost to float — utterly assured and elegant in the otherworldly, treacherously exposed title role.

From New York Times • Jun. 8, 2023

From Frodo’s mind the bright morning — treacherously bright, he thought — had not banished the fear of pursuit; and he pondered the words of Gildor.

From "The Fellowship of the Ring" by J.R.R. Tolkien