Treacherous means dangerous in a way that hides itself. The path across the rocks looks solid until the wet weed slides out from under your boot; the river looks slow, and the pull underneath takes your legs away.
The other half of the word is about people: a treacherous friend smiles at you and sells you out. Both halves share one idea, that the danger sits in the gap between how safe a thing looks and what it does. It goes with weather and ground above all else: treacherous ice, treacherous roads, treacherous waters, treacherous conditions. The noun is treachery and the adverb treacherously, and there is no verb to match, so you betray someone rather than treacher them. Plain danger is not enough for it either: a lit motorway in daylight is dangerous, black ice on an unlit bend is treacherous.
1adjectivetreacherousDangerous in a hidden way, when the ground, the weather or the water looks safer than it is.
“Fresh snow had covered the ice, and the last kilometre was treacherous.”
2adjectivedisloyalBetraying the people who trust you.
“He was hanged as a treacherous servant who had opened the gate at night.”
synonyms: disloyal, perfidious
Treacherous came from Old French trecheros, which meant deceitful, so the word was about lying long before it was about ice. English built it on treacher, an old name for a cheat, with the ending -ous that turns a thing into a quality. The slippery road borrowed its name from the liar.
Old French
trecheros
English, today
treacherous
Built from
From Old French trecheros, tricheros (“deceitful”), equivalent to treacher + -ous. See treacher.
Etymology from Wiktionary, CC BY-SA
Practised in Sharper Adjectives, A Man Called Ove
First appears in chapter 9 of A Man Called Ove.
Practise this word