The Collaborative International Dictionary
Insidious \In*sid"i*ous\, a. [L. insidiosus, fr. insidiae an ambush, fr. insidere to sit in; pref. in- + sedere to sit: cf. F. insidieux. See Sit.]
Lying in wait; watching an opportunity to insnare or entrap; deceitful; sly; treacherous; -- said of persons; as, the insidious foe. ``The insidious witch.''
--Cowper.-
Intended to entrap; characterized by treachery and deceit; as, insidious arts.
The insidious whisper of the bad angel.
--Hawthorne. -
Acting or proceeding unobserved or in a seemingly harmless manner, but slowly or eventually doing great damage; as, an insidious disease; an insidious plot.
Insidious disease (Med.), a disease existing, without marked symptoms, but ready to become active upon some slight occasion; a disease not appearing to be as bad as it really is.
Syn: Crafty; wily; artful; sly; designing; guileful; circumventive; treacherous; deceitful; deceptive. -- In*sid"i*ous*ly, adv. -- In*sid"i*ous*ness, n.
Wiktionary
adv. in an insidious manner
WordNet
adv. in a harmfully insidious manner; "these drugs act insidiously" [syn: perniciously]
Usage examples of "insidiously".
Next, insidiously and quietly, the cartels would become stronger than the governments-after which, as in Colombia, there was never any turning back.
As she's telling what she sees as etiological truth, even though the monologue seems sincere and unaffected and at least a B+ on the overall AA-story lucidity-scale, faces in the hall are averted and heads clutched and postures uneasily shifted in empathetic distress at the look-what-happened-to-poor-me invitation implicit in the tale, the talk's tone of self-pity itself less offensive (even though plenty of these White Flaggers, Gately knows, had personal childhoods that made this girl's look like a day at Six Flags Over the Poconos) than the subcurrent of explanation, an appeal to exterior Cause that can slide, in the addictive mind, so insidiously into Excuse that any causal attribution is in Boston AA feared, shunned, punished by empathic distress.
The beats that slid insidiously from the speakers were all at the high end, cymbals, no bassline.
For myself, I feel it as a background, a background which then insidiously foregrounds itself.
Elwood had not ceased to think of her as of some good angel, sent by an interposing Providence, in answer to the agonizing supplications which immediately preceded her unexpected appearance at the time,—sent to be the means, in some unforeseen way, of extricating her family from the fatal influences, as she viewed them, under which they had insidiously been brought by their different connections with the Gurleys.
Tendrils of the Oneness reached out insidiously to penetrate her mind, to reach down into the nerve centers of her brain.
The demand from network television for a quick, witty spray of set-up-punch-line fare seemed designed, however insidiously, to abbreviate anything approaching actual social commentary.
One sure lesson she had learned from her years among the men was to always hold some portion of herself in reserve, the transparency of womanhood in this insidiously surveillant society requiring here and there certain opaque gaps in order to maintain even minimal standards of sanity.
Her fingers were so very slim and silken dry, so very strong and many, all starting to grip tightlythey were not fingers but wiry black vines rooted inside her skull, growing in profusion out of her cavernous orbits, gushing luxuriantly out of the triangular hole between the nasal and the vomer bones, twining in tendrils from under her upper teeth so white, pushing insidiously and insistently, like grass from a sidewalk crack, out of her pale brown cranium, bursting apart the squamous, sagittal, and coronal sutures.