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Pernicious

1521, Middle English, from M.Fr. pernicios (13c., Fr. pernicieux),
from L. perniciosus "destructive," from pernicies "destruction,
death, ruin," from per- "completely" + necis "violent death, murder,"
related to necare "to kill," nocere "to hurt, injure, harm," noxa "harm,
injury"

-adjective
1. causing insidious harm or ruin; ruinous; injurious;
hurtful: pernicious teachings; a pernicious lie.
2. deadly; fatal: a pernicious disease.
3. Obsolete. evil; wicked.

Information source - Online Etymology Dictionary