pernicious

Etymology 1

From Middle English, from Old French pernicios, from Latin perniciōsus (“destructive”), from perniciēs (“destruction”), from per (“through”) + nex (“slaughter, death”).

adj

1. Causing much harm in a subtle way.
   Puritanism no longer employs the thumbscrew and lash; but it still has a most pernicious hold on the minds and feelings of the American people. 1911, Emma Goldman, “The Hypocrisy of Puritanism”, in Anarchism and Other Essays
   In January, the company laid off a third of its staff and renounced ads as a pernicious influence on the world, without mentioning that Google and Facebook are so good at ads there’s hardly room for anyone else to compete. Mar 22, 2017, Jacob Kastrenakes, “Medium launches memberships for $5 per month”, in The Verge
   A man has always been a woman’s best excuse to avoid her destiny; that a man is her destiny is one of patriarchy’s most pernicious tenets. What a scam. 2019-07-09, Toni Bentley, “What Do the Sex Lives of ‘Three Women’ Tell Us About Female Desire?”, in New York Times
2. Causing death or injury; deadly.

Etymology 2

Analyzable as pernīx ("swift") + -ious.

adj

1. (obsolete) swift; celeritous.