An example of insidious is a plan that is meant to scam people out of money.
Insidious rumors; an insidious disease.
An insidious disease.
Origin of insidious
From American Heritage Dictionary of the English Language, 5th Edition
- 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”).