To save this word, you'll need to log in.
Definition of insidious
1a : 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
2a : awaiting a chance to entrap : treacherous
b : harmful but enticing : seductive insidious drugs
Keep scrolling for more
Keep scrolling for more
Keep scrolling for more
Keep scrolling for more
Test Your Vocabulary
Great Scrabble Words—A Quiz
- Which of the following Q-without-U words means the number five in cards or dice?

Can you spell these 10 commonly misspelled words?
TAKE THE QUIZ
Test Your Knowledge - and learn some interesting things along the way.
TAKE THE QUIZLove words? Need even more definitions?
Merriam-Webster unabridgedWords at Play
Ask the Editors
Word Games
Great Scrabble Words—A Quiz
Words to improve your Scrabble game
Take the quizSemantic Drift Quiz
A challenging quiz of changing words.
Take the quizSpell It
Can you spell these 10 commonly misspelled words?
Take the quizDictionary Devil
The dictionary has been scrambled—can you put it back together?
Play the game
Share insidious
Time Traveler for insidious
The first known use of insidious was in 1545
See more words from the same year