Definition of insidious
1a
: having a gradual and cumulative effect : subtle
the insidious pressures of modern life
b
of a disease
: developing so gradually as to be well established before becoming apparent
2a
: awaiting a chance to entrap : treacherous
b
: harmful but enticing : seductive
insidious drugs
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Merriam-Webster unabridged
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Time Traveler for insidious
The first known use of insidious was in 1545
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