Pernicious
Etymology
From Latin “perniciosus,” destructive, from “pernicies,” ruin or destruction, from “per-” (thoroughly) and “nex,” death or slaughter (the root behind “necro-”). The word carries a whiff of the lethal: what is pernicious does not merely trouble, it destroys.
adjective · /per-NISH-uhs/
Highly destructive or harmful, especially in a way that is deep and lasting.
What it names
Serious, ruinous harm — the strong word for damage that goes to the root rather than the surface.
Example
The pernicious idea that rest is laziness had wrecked his health before he questioned it.
Related idea
Something pernicious is deadly in effect; something insidious is stealthy in method. A thing can be both — quietly working (insidious) toward ruin (pernicious).