insidious
- 1Doing harm in a stealthy, often gradual way that is hard to notice.
The disease is insidious, progressing for years before any symptoms appear.
Small daily compromises had a more insidious effect on the team's morale than any single defeat.
Similarsubtle
Note
The Latin root pictures an ambush: īnsidiae meant a lying in wait, from a verb meaning "to sit in", so what is insidious waits unseen before it does its damage. Medical writing uses the word for a disease, a cancer or an infiltration that advances quietly without obvious signs. Subtle is a near-synonym, but subtle need not mean harmful; insidious always does.
- 2Designed to entrap, appearing attractive while being harmful.
Cult recruiters use insidious flattery to draw lonely people in.
The ad's insidious promise of quick riches hid the fees buried in the contract.
Similarseductive, beguiling, deceptive
Note
Here the ambush wears a friendly face: the harm hides behind something inviting, such as flattery, a whisper or a handshake. Dickens gives Gashford a "cold insidious palm" to slide into his master's grasp in Barnaby Rudge (1841), and Hawthorne writes of the insidious whispers of the bad angel in The Scarlet Letter (1850). The word points at the bait rather than at slow damage.
- 3Treacherous and disloyal, ready to betray trust or allegiance.
The general blamed the defeat on a handful of insidious officers who had switched sides.
The rebels never forgave their insidious allies, who sold their plans to the enemy.
Similartreacherous, disloyal, traitorous
Note
Calling people insidious simply because they betray is nonstandard. Careful writers keep the word for harm that works by stealth and reach for treacherous or traitorous when someone switches sides. The looser use usually targets enemies, defectors and rival powers, often in heated speech, as when Phineas Camp Headley's life of the Empress Josephine calls Austria "that insidious house".
Practice
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Origin
From Middle French insidieux, from Latin īnsidiōsus ("cunning, artful, deceitful"), from īnsidiae ("a lying in wait, an ambush, stratagem"), from īnsideō ("to sit in or on"), from in ("in, on") + sedeō ("to sit").