Working or spreading harmfully in a subtle or stealthy manner: insidious rumors; an insidious disease.
Intended to entrap; treacherous: insidious misinformation.
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 Indo-European roots.] in·sid'i·ous·ly adv., in·sid'i·ous·ness n.
1545, from L. insidiosus "deceitful," from insidiæ (pl.) "plot, snare, ambush," from insidere "sit on, occupy," from in- "in" + sedere "to sit" (see sedentary).
Main Entry: in·sid·i·ous Pronunciation: in-'sid-E-&s Function: adjective : developing so gradually as to be well established beforebecoming apparent insidious disease> —in·sid·i·ous·lyadverb