insidious {adjective} = Proceeding in a gradual, subtle way, but with very harmful effects:

Etymonline: 1540s, from Middle French insidieux (15c.) or directly from Latin insidiosus "deceitful, cunning, artful," from insidiae (plural) "plot, snare, ambush," from insidere "sit on, occupy," from in- "in" (see in- (2)) + sedere "to sit" (see sedentary). Related: Insidiously; insidiousness.)

I wish to delve into the definition, which I already understand and so ask NOT about. I heed the Etymological Fallacy. What are some right ways of interpreting this meaning (to naturalise or rationalize it), so that it feels reasonable and intuitive?

Here are my thoughts. If a problem is sitt[ing] in your awareness or domain, then it already disturbs you instantly. So what's gradual, subtle ?

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    There were probably 1000 years between Latin insidiae and French insidieux. This gives time for a much more radical change in meaning than the one you see here. I think it's pretty clear why sit in meant ambush. And one route to the present meaning could be ambushinfiltratorinsidious. Mar 25, 2015 at 16:01
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    Your presupposition, "it already disturbs you instantly", is just your opinion. The word means something more akin to "sitting patiently/unobtrusively in order to be overlooked so that one can do damage without being detected*. I don't understand your questions sometimes. It seems that you just want to disagree with language. You might get more satisfaction from ripping pages out of your dictionary and setting them on fire. Mar 25, 2015 at 16:02

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What could be more gradual than sitting quietly, waiting for an opportunity to strike? Human imagination plays a huge role in the etymology of words:

Imagine a heard of gazelle munching grass on the Serengetti. They have no idea that just 50 yards away, an insidious lion is crouching quietly in the grass, waiting for them to cross the imaginary line that makes them lunch.

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Imagine the insidious varicella-zoster virus, sitting quietly in the nerve tissue at the base of your brain--from the time you had chicken pox at age 3 until today--waiting for a lapse in your immune system to break out in a painful attack of shingles!

Nothing in the etymology suggests sitting in plain sight, although some of the insidious dangers for children do sit in plain sight: the plastic bags infants suffocate in, and the pools children drown in.

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    Nice visual. Very insidious-looking! Mar 25, 2015 at 19:32