'arapaima' originates from Portuguese, ultimately from the Tupi word 'arapaíma', where it referred to a large Amazonian freshwater fish.

'arapaima' entered English from Portuguese (via reports and natural history accounts of the 18th–19th centuries) as 'arapaima' and has been used in English to name the species.

Initially it meant the local Tupi/Portuguese name for the fish; over time it has retained that specific meaning in English as the name of the large Amazonian freshwater fish.