Main Entry: in·sid·i·ous

Pronunciation: \in-ˈsi-dē-əs\

Function: adjective

Etymology: Latin insidiosus, from insidiae ambush, from insidēre to sit in, sit on, from in- + sedēre to sit — more at sit

Date: 1545

1 a : awaiting a chance to entrap : treacherous b : harmful but enticing : seductive <insidious drugs>
2 a : having a gradual and cumulative effect : subtle <the insidious pressures of modern life> b of a disease : developing so gradually as to be well established before becoming apparent

— in·sid·i·ous·ly adverb

— in·sid·i·ous·ness noun