insidious

The story

1. Latin · derived fromīnsidiōsus“cunning, artful, deceitful”
2. Middle French · derived frominsidieux
3. English todayinsidious

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”).