Treacherous
adjective
Definition
1. (adjective) Not loyal and willing to betray trust — describing a person who deceives or stabs others in the back.
2. (adjective) Dangerous and unpredictable — used for conditions, surfaces, roads, or weather that can suddenly cause harm.
2. (adjective) Dangerous and unpredictable — used for conditions, surfaces, roads, or weather that can suddenly cause harm.
Context Alive
You're driving home from a weekend trip and the weather changes fast. One minute the road is clear, the next it's covered in black ice. The car in front of you slides sideways and almost hits the barrier. You slow down to a crawl and grip the steering wheel. A sign flashes: "Caution — treacherous road conditions ahead." You pull over at the next service station and decide to wait it out.
Meanings
2 meanings 1 Disloyal and Willing to Betray (Adjective) Common ▼
This meaning is about a person who pretends to be trustworthy but is actually willing to betray you — someone dangerous because you don't see the betrayal coming. Imagine a colleague who smiles to your face, agrees with everything you say in meetings, but behind your back tells the boss your ideas are terrible and takes credit for your work — that person is treacherous. This is describing someone whose disloyalty is hidden under a surface of friendliness or trust. You might read "the treacherous adviser secretly worked for the enemy" in a novel about political betrayal, or someone could say "be careful with him — he's treacherous" as a warning about a person who can't be trusted. Or think about a historical king whose most trusted general switched sides during battle — that act was treacherous because it came from someone who was supposed to be loyal. The word suggests betrayal that's hidden, deliberate, and deeply harmful.
✏️ Treacherous is stronger than "disloyal" or "untrustworthy." It implies real danger — a treacherous person doesn't just let you down, they actively work against you while pretending to be on your side. The noun form is treachery: "his treachery cost them the war." You'll see this word a lot in historical writing, political drama, and fantasy novels.
2 Dangerous and Unpredictable — Conditions (Adjective) Very Common ▼
This meaning is about physical conditions that are dangerous because they're hard to predict or because they can change suddenly — roads, weather, terrain, water. Imagine hiking in the mountains and the path suddenly narrows to a thin ledge with loose rocks and a steep drop on one side — the conditions are treacherous because one wrong step could be fatal. This is describing danger that isn't always obvious — it can catch you off guard. You might hear "treacherous road conditions" on a weather report warning drivers about ice or flooding, or someone could say "the sea looks calm but the currents are treacherous" about water that's more dangerous than it appears. Or think about a mountain pass in winter where the road is covered in black ice — it looks fine but your car can lose control in a second. The word suggests danger that's partly hidden or deceptively calm on the surface.
✏️ This is the meaning you'll hear most often in everyday life — especially in weather and traffic reports. "Treacherous conditions" is almost a fixed phrase in British English. The key idea is that the danger isn't always visible: the road looks fine but it's icy, the sea looks calm but the currents are deadly. That hidden quality is what makes something treacherous rather than just dangerous.
Common Patterns
Basic Structures
treacherous + noun (conditions/roads/waters) → the most common pattern — describing dangerous physical environments
Drivers are warned of treacherous road conditions across the north of England tonight.
treacherous + noun (person/ally/adviser) → describing someone who betrays trust
The king's most treacherous adviser had been passing secrets to the enemy for years.
prove treacherous → to turn out to be dangerous or disloyal
The mountain path proved treacherous after the rain — two hikers had to be rescued.
Common Structures
treacherous conditions → a near-fixed phrase for dangerous weather or road situations
The match was cancelled due to treacherous conditions on the pitch.
notoriously treacherous → widely known for being dangerous
That stretch of coastline is notoriously treacherous — dozens of ships have sunk there.
treacherous act / act of treachery → a specific betrayal or disloyal action
Selling the company's secrets was the most treacherous act anyone could imagine.
Collocations
10 collocationstreacherous conditions
dangerous and unpredictable weather or road situations
treacherous roads
roads that are dangerously icy, wet, or unstable
treacherous waters
sea or river conditions that are deceptively dangerous
treacherous terrain
ground or landscape that is dangerous to cross
treacherous weather
weather that changes suddenly and creates danger
treacherous ally
someone who pretends to be on your side but will betray you
treacherous act
a specific act of betrayal or disloyalty
notoriously treacherous
famous for being dangerous or untrustworthy
prove treacherous
to turn out to be more dangerous than expected
treachery and betrayal
a common pairing in storytelling — acts of disloyalty
Example Sentences
10 examples
1
The roads are treacherous this morning — there's black ice everywhere.
Driving is extremely dangerous this morning because of hidden ice on the roads.
2
He turned out to be a treacherous friend who shared her secrets with everyone.
He proved to be completely disloyal — he told everyone the things she had trusted him with.
3
The river may look calm, but the currents underneath are treacherous.
The river seems peaceful on the surface, but the hidden currents are extremely dangerous.
4
The climb became treacherous after dark — the rocks were wet and impossible to grip.
Once night fell, the climb turned dangerously difficult — the wet rocks offered no grip at all.
5
History remembers him as a treacherous general who switched sides at the worst possible moment.
History records him as a disloyal commander who betrayed his own side when they needed him most.
6
Police have warned drivers to avoid the motorway due to treacherous conditions.
Police are telling drivers to stay off the motorway because the conditions are dangerously unsafe.
7
The political landscape was treacherous — you never knew who was really on your side.
The political environment was full of hidden dangers — trust was impossible to rely on.
8
That stretch of coastline is notoriously treacherous for sailors.
That part of the coast is well known for being extremely dangerous to anyone at sea.
9
She survived the treacherous mountain pass by walking slowly and roping herself to the rock face.
She made it through the deadly mountain pass by moving carefully and tying herself to the cliff.
10
His treachery was eventually discovered, and he was removed from the council.
His betrayal was finally uncovered, and he was kicked out of the council.
Synonyms & Antonyms
6 items
Synonymsdangerous
more general and everyday — doesn't carry the hidden or deceptive quality
disloyal
specifically about people — less dramatic and intense than treacherous
perilous
formal — focuses on extreme danger, often for journeys or situations
Antonymsloyal
the direct opposite for people — faithful, trustworthy, and reliable
safe
the opposite for conditions — no hidden dangers or risks
trustworthy
someone you can rely on completely — the opposite of a treacherous person