insidious
adj.
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.
〔李〕[in-=in内；sid =to sit坐；-i-;-ous a.=like像……的→“like sitting in a hidden place, like setting an ambush像坐在暗处的，像打埋伏的”→] a.① intended to entrap伺机为害的