'Pernicious' is Latin for 'completely killing' — from 'per-' (completely) + 'necare' (to kill). Deadly quiet.
Having a harmful effect, especially in a gradual or subtle way.
From Latin 'perniciōsus' (destructive, ruinous, deadly), from 'perniciēs' (ruin, destruction, death), a compound of 'per-' (completely, thoroughly — an intensifying prefix) + 'nex' (genitive 'necis,' violent death, slaughter), from the PIE root *neḱ- (death, corpse, to kill). PIE *neḱ- is one of the richest roots for mortality in Indo-European: it produced Latin 'necare' (to kill → 'internecine,' mutual slaughter), 'nex' (violent death), and 'nectar' (the drink of immortality — literally 'not-death' from *neḱ- + *-tro, a negating suffix). Greek reflexes include 'nekros' (corpse → 'necropolis