Definition of insidious in English:

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Pronunciation: /inˈsidēəs/


1Proceeding in a gradual, subtle way, but with harmful effects: sexually transmitted diseases can be insidious and sometimes without symptoms
More example sentences
  • Nuclear disc lesions are of gradual or insidious onset, the history may be on and off back pain for weeks and back pain getting worse.
  • The obstruction and harassment is subtle but insidious and seriously affects the ability of the aid agencies to do their job.
  • It need by no means be obvious: it can be insidious and subtle.
stealthy, subtle, surreptitious, cunning, crafty, treacherous, artful, sly, wily, shifty, underhanded, indirect
informal sneaky
1.1Treacherous; crafty: tangible proof of an insidious alliance
More example sentences
  • What they do is insidious, evil, and downright mean to their addict-victims like my wife.
  • They are, in my view, the most insidious of traitors.
  • Personally, we can't stand this insidious practise of sneaking adverts into non-commercial formats.



Pronunciation: /inˈsidēəsnəs/
Example sentences
  • But for five Irish teenagers, the magnifying glass doesn't reveal the insidiousness of California - it only inflates its lure and significance.
  • Slowly, with an insidiousness no one recognized, those businesses began to disappear until now many of the buildings sit empty and abandoned.
  • Asthma has become a common ailment in modern society, as the deteriorating environment and stress work their insidiousness against the human body.


Mid 16th century: from Latin insidiosus 'cunning', from insidiae 'an ambush or trick', from insidere 'lie in wait for', from in- 'on' + sedere 'sit'.

Words that rhyme with insidious

fastidious, hideous, invidious, perfidious

For editors and proofreaders

Syllabification: in·sid·i·ous

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