ob-, on, with emphasis, -nox, to injure, to kill, 語源的にはnoxious, necrosisと同じ。
Obnoxious has two very different senses, one of which (exposed or open or liable to attack or injury) requires notice because its currency is now so restricted that it is puzzling to the uninstructed. It is the word's rightful or de jure meaning, and we may hope that scholarly writers will keep it alive. [Fowler]Related: Obnoxiously; obnoxiousness.