pernicious (adj.)

early 15c., of a deed, "evil, wicked;" from 1520s as "having the property of destroying or being injurious," from Old French pernicios (13c., Modern French pernicieux) and directly from Latin perniciosus "destructive," from pernicies "destruction, death, ruin," from per "completely" (see per) + necis "violent death, murder," related to necare "to kill," nocere "to hurt, injure, harm," noxa "harm, injury" (from PIE root *nek- (1) "death"). Related: Perniciously; perniciousness.