- insidious (adj.)
- 1540s, from Middle 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" (see in- (2)) + sedere "to sit" (see sedentary). Figurative, usually with a suggestion of lying in wait and the intent to entrap. Related: Insidiously; insidiousness.
