Insidious means doing harm gradually and without being noticed. An insidious disease, insidious effects, insidious propaganda.
The hidden part is the whole word: an insidious illness has no early symptoms, which is exactly what makes it dangerous. It is not the same as invidious, which means likely to cause resentment, and the two are constantly confused.
1adjectiveinsidiousDoing harm gradually and without being noticed.
“Rust is an insidious enemy of old cars, eating away beneath the paint.”
insidious is a quality.
Latin insidiosus means cunning, from insidiae, an ambush, from insidere, to sit in wait.
Proto-Indo-European
*sed-
Latin
īnsidiōsus — cunning, artful, deceitful
Middle French
insidieux
English, today
insidious
Built from
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”).
Etymology from Wiktionary, CC BY-SA