The story
Coined in a naming competition. Like its Chinese counterpart 八達通 /八达通 (baat³ daat⁶ tung¹), it references the number eight, as an octopus has eight tentacles.
Etymology from Wiktionary, CC BY-SA 4.0.
Octopus card
Today it means: A contactless smart card, introduced in 1997, used as a form of payment in Hong Kong, originally for the mass transit system, but now widely used for public transport and other retail transactions. Full definition →