Definition of "insidious"

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.

2023 March 8, Gareth Dennis, “The Reshaping of things to come...”, in RAIL, number 978, page 49

Gashford slid his cold insidious palm into his master's grasp, and so, hand in hand, and followed still by Barnaby and by his mother too, they mingled with the concourse.

1841 February–November, Charles Dickens, “Barnaby Rudge”, in Master Humphrey’s Clock, volume III, London: Chapman & Hall, […], chapter 48, page 215

The insidious whispers of the bad angel.

1850, Nathaniel Hawthorne, chapter 5, in The Scarlet Letter, a Romance, Boston, Mass.: Ticknor, Reed, and Fields

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., 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

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