'Arapaima' originates from Portuguese, specifically the word 'arapaíma', which itself comes from languages of the Amazon region (notably Tupi).

'Arapaima' entered English from Portuguese 'arapaíma' (borrowed from Tupi or related indigenous names for the fish), and the form was adopted into English largely in the 19th century through natural history and exploration accounts.

Initially it was a local name for the fish in Indigenous and Portuguese usage; over time it became the standard English name for the large Amazonian fish species group 'Arapaima'.