Insidious
mid 16th century: from Latin insidiosus ‘cunning’, from insidiae ‘an ambush or trick’, from insidere ‘lie in wait for’, from in- ‘on’ + sedere ‘sit’.
wiktionary
From Middle French insidieux, from Latin īnsidiōsus(“cunning, artful, deceitful”), from īnsidiae(“a lying in wait, an ambush, artifice, stratagem”) + -ōsus, from īnsideō(“to sit in or on”), from in(“in, on”) + sedeō(“to sit”).
etymonline
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.
