1630s, "noxious, harmful," from Latin nocuus "harmful," from stem of nocere "to hurt, injure, harm" (from PIE root *nek- (1) "death"). Especially of venomous serpents.
Entries linking to nocuous
*nek- 1
Proto-Indo-European root meaning "death." It forms all or part of: innocent; innocuous; internecine; necro-; necropolis; necrosis; necromancy; nectar; nectarine; nociceptive; nocuous; noxious; nuisance; obnoxious; pernicious.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit nasyati "disappears, perishes," Avestan nasyeiti "disappears," nasu- "corpse," Old Persian vi-nathayatiy "he injures;" Greek nekros "corpse;" Latin nex, genitive necis "violent death, murder" (as opposed to mors), nocere "to harm, hurt," noxius "harmful;" Greek nekus "dead" (adj.), nekros "dead body, corpse;" Old Irish ec, Breton ankou, Welsh angeu "death."