Learn English – Difference between ‘Insidious’ and ‘Pernicious’

meaningword-choiceword-usage

Do the words 'insidious' and 'pernicious' have the same meaning? And can they be used in place of each other?

On some places on the web (eg : http://www.dailywritingtips.com/insidious-vs-invidious/), it says that the two words have different uses, but the oxford dictionary defines them both similarly.

Oxford dictionary links : insidious and pernicious

Best Answer

I'll go again with Merriam-Webster 3rd International:

insidious:

  1. watching for an opportunity to ensnare: insidious tempter
  2. intended to entrap or trick: insidious plot
  3. acting by imperceptible degrees: insidious disease
  4. having gradual, cumulative, and often hidden effects; insidious pressures of modern life
  5. subtle: insidious charm

For synonyms it gives 'sly', but obviously that won't work everywhere, mostly 1, 2, possibly 5.

Pernicious has none of these meanings. It basically means destructive or harmful. in the names of diseases (pernicious malaria, pernicious anemia) it means especially severe forms of the disease.

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