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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
I'll go again with Merriam-Webster 3rd International:
insidious:
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.
Insidious suggests a lying in wait or a gradualness of effect or approach and applies especially to devious and carefully masked underhandedness.
Pernicious is more often applied to things that harm exceedingly or irrepairably by evil or by insidious corrupting.
(excerpts from MW dictionary of synonyms)
The actual OED does not make them synonyms, defining "pernicious" as extremely harmful (from the Latin per (intensive prefix) + nex, necis death) and "insidious" as treacherous (from the Latin insidere, to lie in wait). Ambushers lying in wait for you can't intend anything but harm, but they needn't mean to kill you. And something can be pernicious and readily apparent. Consider the sentence from The Province of Jurisprudence by John Austin:
What appears pernicious to one person may appear beneficial to another.
That doesn't mean that the two can't go together. Pernicious anemia was named before its cause was understood, at a time when it was invariably fatal. But it is also an insidious disease, developing slowly with multiple symptoms that evade an easy diagnosis.
Good research. Your first reference does a good job of pointing out the subtle distinctions. In particular, it points out that "insidious" is
The Oxford dictionary does define them similarly, but not identically:
for "insidious", notice that it mentions both the gradual nature and the harmful effect together.
For "pernicious", it mentions first the harmful effect and then says "especially" about the gradualness.
So according to Oxford Dictionaries:
"insidious" things always come on gradually and always have harmful effects.
"pernicious" things are always harmful**, and often (but not necessarily) come on gradually.
So you could use either word for things that are very hamful and come on gradually or subtly.
However, for something that is only, say, annoying (like that song you just can't get out of your head) you could say it is "insidious", but not "pernicious". It insinuates itself, not innocently, but innocuously.
Insidious an insidious change or problem spreads gradually without being noticed, and causes serious harm
an insidious trend towards censorship of the press
Pernicious very harmful or evil, often in a way that you do not notice easily
the pernicious effects of poverty.
Pernicious is just a more severe and harmful form of insidious. Something that is insidious in nature may or may not be pernicious. The bermuda grass may be taking over my yard insidiously, but i would not say it is pernicious. I would save pernicious for some diseases that come on gradually but have severe consequences.
