Pernicious means harmful and subtle, such as a poison gas that causes illness in those exposed to it over the course of years.

Pernicious comes from the Latin perniciosus, "destructive," which in turn comes from pernicies, "death" or "ruin." You might have heard your parents and teachers talk about the pernicious effects of watching too much TV and playing video games all day — they'll turn your brain to mush (allegedly).

Definitions of pernicious

adj exceedingly harmful

baneful, deadly, pestilent
injurious to physical or mental health

adj working or spreading in a hidden and usually injurious way

insidious, subtle
causing or capable of causing harm

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