Having a harmful effect in a gradual or subtle way; destructive in a way that may not be immediately obvious.
From Latin 'perniciosus,' from 'pernicies' meaning 'destruction,' from 'per-' (thoroughly) and 'nex' (death). The word suggests complete destruction that works slowly and insidiously.
Think 'per-nicious' as 'per-vicious'—thoroughly vicious but in a sneaky way. Like carbon monoxide, pernicious things are dangerous precisely because their harm isn't immediately apparent.
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