insidious (adj.) Look up insidious at Dictionary.com
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.