Insidious

//ɪnˈsɪdi.əs// adj

adj ·Common ·High school level

Definitions

Adjective
  1. 1
    Causing harm in a stealthy, often gradual, manner.

    "Strong and vigorous man as he looks, Livingstone has been for years the victim of a secret and insidious disease."

  2. 2
    Intending to entrap; alluring but harmful.

    "Hansel and Gretel were lured by the witch’s insidious gingerbread house."

  3. 3
    Treacherous. nonstandard

    "The battle was lost due to the actions of insidious defectors."

Adjective
  1. 1
    working or spreading in a hidden and usually injurious way wordnet
  2. 2
    intended to entrap wordnet
  3. 3
    beguiling but harmful wordnet

Synonyms

All synonyms
dangerousacuteamoralarchartfulastute

Example

More examples

"In junior high and high schools, they say insidious forms of bullying are on the rise."

Etymology

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”).

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Data sourced from Wiktionary, WordNet, CMU, and other open linguistic databases. Updated March 2026.