Definition of insidious

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in·sid·i·ous    (n-sd-s) KEY 

ADJECTIVE:

1. Working or spreading harmfully in a subtle or stealthy manner: insidious rumors; an insidious disease.
2. Intended to entrap; treacherous: insidious misinformation.
3. Beguiling but harmful; alluring: insidious pleasures.

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ETYMOLOGY:
From Latin nsidisus, from nsidiae, ambush, from nsidre, to sit upon, lie in wait for : in-, in, on ; see in-² + sedre, to sit; see sed- in Indo-European roots

OTHER FORMS:
in·sidi·ous·ly(Adverb), in·sidi·ous·ness(Noun)