The story
From octo- + bass. The name "octobass" comes from "octo-" meaning "eight" in Latin and "-bass", which is general name for any bass instrument (like "double bass", "contrabass" etc.). "Octo" is used because its range extending more than an octave below that of the double bass - it is tuned to A0, compared to the C1 that a double bass can stretch to, which makes octobass the lowest chordophone ever created.
Etymology from Wiktionary, CC BY-SA 4.0.