Pernicious things (and people) are evil, deadly, or destructive, often in a way that's sneaky or gradual.
per NISH us
Part of speech:
(Adjectives are describing words, like "large" or "late."
They can be used in two ways:
1. Right before a noun, as in "a pernicious thing" or "a pernicious person."
2. After a linking verb, as in "It was pernicious" or "He was pernicious.")
Other forms: Perniciously & perniciousness.
(Although there's a nicer-sounding noun, "pernicion," no one uses it now.)
How to use it:
This word is formal and very negative.
Talk about pernicious diseases and insects; pernicious problems and issues; pernicious threats and weaknesses; pernicious crimes, actions, practices, and campaigns; pernicious intents and purposes; pernicious ideas, doctrines, principles, and ideologies; pernicious superstitions and conspiracy theories; pernicious apathy and ignorance; pernicious influences and effects; pernicious cycles of events, and so on.
One thing can be pernicious to another: "a disease pernicious to these plants," "a shyness pernicious to his social life."
It's much less common, but you can also say that people are pernicious.
examples: Flint's water crisis worsened in the face of pernicious neglect.
A promise to love, honor, and obey? Plenty of us omitted that pernicious "obey" from our wedding vows.