in·sid·i·ous 
(ĭn-sĭdē-əs)
(ĭn-sĭdē-əs)adj.
1. Working or spreading harmfully in a subtle or stealthy manner: insidious rumors; an insidious disease.
2. Intended to entrap; treacherous: insidious misinformation.
3. Beguiling but harmful; alluring: insidious pleasures.
[From Latin īnsidiōsus, from īnsidiae, ambush, from īnsidēre, to sit upon, lie in wait for : in-, in, on; see IN-2 + sedēre, to sit; see sed- in the Appendix of Indo-European roots.]
in·sidi·ous·ly adv.
in·sidi·ous·ness n.