Etymology
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insidious (adj.)

1540s, from French insidieux "insidious" (15c.) or directly from Latin insidiosus "deceitful, cunning, artful, treacherous," from insidiae (plural) "plot, snare, ambush," from insidere "sit on, occupy," from in- "in" (from PIE root *en "in") + sedere "to sit," from PIE root *sed- (1) "to sit." Figurative, usually with a suggestion of lying in wait and the intent to entrap. Related: Insidiously; insidiousness.

updated on December 07, 2020

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Definitions of insidious from WordNet

insidious (adj.)
beguiling but harmful;
insidious pleasures
insidious (adj.)
intended to entrap;
insidious (adj.)
working or spreading in a hidden and usually injurious way;
glaucoma is an insidious disease
Synonyms: pernicious / subtle
Etymologies are not definitions. From wordnet.princeton.edu, not affiliated with etymonline.