1. highly injurious or destructive.
2. archaic: wicked.
– perniciously, adverb.
– perniciousness, noun.
Pernicious implies irreparable harm done through evil or insidious corrupting or undermining.
[Origin: Middle English, from Anglo-French, from Latin perniciosus, from pernicies destruction, from per – + nec -, nex violent death.] 15th Century.
“I found this fellow at the bottom of the chest under a lot of other stuff. Fortunately our pernicious vicar hadn’t quite got to it before his collapse.” The Ghosts of Sleath, James Herbert.