'arapaima' originates from Portuguese, ultimately from the Tupi-Guarani word 'arapaíma', where it meant 'large fish'.

'arapaima' was adopted into Portuguese from Tupi-Guarani 'arapaíma' and later borrowed into English as 'arapaima'.

Initially, it meant 'large fish' as a local common name; over time it retained that sense and now denotes the species (often 'Arapaima' or 'Arapaima gigas').