insidious

Etymology

From Middle French insidieux, from Latin īnsidiōsus (“cunning, artful, deceitful”), from īnsidiae (“a lying in wait, an ambush, artifice, stratagem”) + -ōsus, from īnsideō (“to sit in or on”), from in (“in, on”) + sedeō (“to sit”).

adj

1. Producing harm in a stealthy, often gradual, manner.
   Strong and vigorous man as he looks, Livingstone has been for years the victim of a secret and insidious disease. 1847, George Lippard, The Quaker City: or, The monks of Monk-Hall
   At some point in time they may become the source of an insidious cancer. 1997, Matthew Wood, The book of herbal wisdom: using plants as medicine
   The nurse always must be alert to signs of slow leak or insidious infiltration. 2007, Sharon Weinstein, Ada Lawrence Plumer, Principles and practice of intravenous therapy
   The impact on rural communities of rail closures was acute, but I would argue that the worst outcome it created was the long-term diminishment of suburban capacity outside London, which has had a far more insidious effect on rail usage nationally. March 8 2023, Gareth Dennis, “The Reshaping of things to come...”, in RAIL, number 978, page 49
2. Intending to entrap; alluring but harmful.
   All these facts clearly appear to me now to establish that the sanctioned scheme was a part of a bigger and […] more insidious scheme which was to hoodwink the creditors and to firmly establish and consolidate the position […] 1948, D.V. Chitaley (editor or publisher), All India Reporter, volume 3, page 341
   The atmosphere of this insidious city comes out to meet him the moment he touches the European shore; for in London he meets Maria Gostrey just over from France. 1969, Dorothy Brewster, John Angus Burrell, Dead reckonings in fiction
   They all sought the President's views on the world situation in general and the Asian situation in particular. Without mincing words he would comment on his favorite theme, namely, the insidious scheme of the international Communists to conquer the free world. 1983, James C. H. Shen, “Rejoining the Government”, in Robert Myers, editor, The U.S. & Free China: How the U.S. Sold Out Its Ally, Washington, D.C.: Acropolis Books Ltd., →LCCN, →OCLC, page 28
   This seemed to her the worst defilement into which this insidious city had cheated her and in her agitation, she nearly ran into the latrine, […] 2005, Anita Desai, Voices in the City, page 189
   This is the insidious way sports entrap you: you follow a player, which commits you to his team. You begin to acquire scraps of utterly useless information about teammates, managers, owners, trainers, agents, lawyers. 2007, Joseph Epstein, Narcissus Leaves the Pool, page 171
   Hansel and Gretel were lured by the witch’s insidious gingerbread house.
3. (nonstandard) Treacherous.
   But with whom do you contract that alliance? With the natural enemy of France — that insidious house of Austria — which detests our country from feeling, system, and necessity. 1858, Phineas Camp Headley, The life of the Empress Josephine: first wife of Napoleon
   ‘Believe me,’ he shouted, ‘these insidious folk talk dangerous nonsense. I hear they are spouting out their ridiculous platitudes not five miles from this park in which we are standing…’ 1912, Ralph Straus, The prison without a wall
   The battle was lost due to the actions of insidious defectors.