[in.sid.i.ous] adj [L insidiosus, fr. insidiae ambush, fr. insidere to sit in, sit on, fr. in- + sedere to sit--more at sit] (1545) 1 a: awaiting a chance to entrap: treacherous b: harmful but enticing: seductive "~ drugs"
2. a: having a gradual and cumulative effect: subtle "the ~ pressures of modern life" b of a disease: developing so gradually as to be well established before becoming apparent -- in.sid.i.ous.ly adv -- in.sid.i.ous.ness n