The story
From blue + ringed + octopus. From being an octopus with very prominent bright blue rings all over its body.
Etymology from Wiktionary, CC BY-SA 4.0.
blue-ringed octopus
Today it means: Any member of any species of the genus Hapalochlaena of octopuses, with distinctive blue rings, that carry the highly poisonous venom tetrodotoxin (TTX). Full definition →